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' means one lot of letters goes inside another.
'strategy' becomes 'plan' (a strategy is a plan or tactic).
'too' inserted into '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" .)