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

Ver. 1
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(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
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...
(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!..