Army units also included in the plans (8)
I believe the answer is:
platoons
'army units' is the definition.
(platoon is a kind of army unit)
'also included in the plans' is the wordplay.
'also' becomes 'too' (synonyms).
'included in' indicates putting letters inside.
'too' inserted within 'plans' is 'PLATOONS'.
(Other definitions for platoons that I've seen before include "small bodies of soldiers" , "Military units" , "Army units" .)