Group of soldiers also involved in strategy (7)
I believe the answer is:
platoon
'group of soldiers' is the definition.
(type of military subdivision)
'also involved in strategy' is the wordplay.
'also' becomes 'too' (synonyms).
'involved in' indicates putting letters inside.
'strategy' becomes 'plan' (a strategy is a plan or tactic).
'too' inserted within 'plan' is 'PLATOON'.
(Other definitions for platoon that I've seen before include "Subdivision of a company" , "Group of soldiers" , "Military squad" , "Team under lieutenant" , "Small body of soldiers" .)