Soldiers excessively invested in strategy (7)
I believe the answer is:
platoon
'soldiers' is the definition.
(group of soldiers)
'excessively invested in strategy' is the wordplay.
'excessively' becomes 'too' (to too great a degree).
'invested in' means one lot of letters goes inside another.
'strategy' becomes 'plan' (a strategy is a plan or tactic).
'too' put within 'plan' is 'PLATOON'.
(Other definitions for platoon that I've seen before include "Army unit" , "Military squad" , "Infantry unit" , "Company's subdivision" , "Group of soldiers" .)