Army group also included in proposed idea (7)
I believe the answer is:
platoon
'army group' is the definition.
(I've seen this in another clue)
'also included in proposed idea' is the wordplay.
'also' becomes 'too' (synonyms).
'included in' is an insertion indicator.
'proposed idea' becomes 'plan' (plan is a kind of idea. I am not sure about the 'proposed' bit.).
'too' put within 'plan' is 'PLATOON'.
Can you help me to learn more?
(Other definitions for platoon that I've seen before include "Infantry unit" , "Army unit" , "Company's subdivision" , "Military group" , "detail" .)