Hello.
guix system --version
guix system (GNU Guix) c3bda394a44ceed39c5ac2f1d07502a3a7502da5

Ver. 1
___________________________________________________________________________
(use-modules
 ...
(gnu services mcron)        ;; ← manual
 ...
(define fstrim-job              ;; ← manual
  #~(job "10 10 * * 6"
         "fstrim -a"))
...
(services (cons*
 ...
  (mcron-service-type (mcron-configuration (jobs (list fstrim-job))))  ;; ←
but here the question
 ...

root@guix ~# guix system -n reconfigure /home/jone/sys.cfg/system-config.scm
guix system: error: failed to load '/home/jone/sys.cfg/system-config.scm':
/home/jone/sys.cfg/system-config.scm:123:25:
/home/jone/sys.cfg/system-config.scm:123:25: Wrong type to apply:
#<service-type mcron 227d4b0>

Ver.2
___________________________________________________________________________
 ...
(mcron-service (mcron-configuration (jobs (list fstrim-job))))  ;; ← manual
 ...

root@guix ~# guix system -n reconfigure /home/jone/sys.cfg/system-config.scm
Backtrace:
In srfi/srfi-1.scm:
   592:29 19 (map1 (#<<service> type: #<service-type mingetty 12c…> …))
   592:29 18 (map1 (#<<service> type: #<service-type mingetty 12c…> …))
   592:29 17 (map1 (#<<service> type: #<service-type mingetty 12c…> …))
   592:29 16 (map1 (#<<service> type: #<service-type mingetty 12c…> …))
   592:29 15 (map1 (#<<service> type: #<service-type mingetty 12c…> …))
   592:29 14 (map1 (#<<service> type: #<service-type agetty 12c19…> …))
   592:29 13 (map1 (#<<service> type: #<service-type console-font…> …))
   592:29 12 (map1 (#<<service> type: #<service-type virtual-term…> …))
   592:29 11 (map1 (#<<service> type: #<service-type ntp 12c1320>…> …))
   592:29 10 (map1 (#<<service> type: #<service-type dbus 12c1460…> …))
   592:29  9 (map1 (#<<service> type: #<service-type elogind 2dfe…> …))
   592:29  8 (map1 (#<<service> type: #<service-type upower 2dfee…> …))
   592:29  7 (map1 (#<<service> type: #<service-type avahi 2dfefa…> …))
   592:29  6 (map1 (#<<service> type: #<service-type wpa-supplica…> …))
   592:29  5 (map1 (#<<service> type: #<service-type network-mana…> …))
   592:29  4 (map1 (#<<service> type: #<service-type slim 2dfef50…> …))
   592:17  3 (map1 (#<<service> type: #<service-type mcron 12c14b0>…>))
In gnu/services.scm:
   777:22  2 (loop #<<service> type: #<service-type mcron 12c14b0> v…>)
In gnu/services/mcron.scm:
    99:33  1 (extend #<<mcron-configuration> mcron: #<package mcron…> …)
In unknown file:
           0 (append #<<mcron-configuration> mcron: #<package mcron…> …)

ERROR: In procedure append:
ERROR: In procedure append: Wrong type argument in position 1 (expecting
empty list): #<<mcron-configuration> mcron: #<package mcron@1.1.1
gnu/packages/guile.scm:683 260f600> jobs: (#<gexp (job "10 10 * * 6"
"fstrim -a") 31b7630>)>

Questions!..