A trooper's first strategy — charge! (6)
I believe the answer is:
attack
'charge' is the definition.
(I've seen this in another clue)
'a trooper's first strategy' is the wordplay.
'trooper's first' becomes 't' (1st letter of 'trooper').
'strategy' becomes 'tack' (I've seen this in another clue).
'a'+'t'+'tack'='ATTACK'
(Other definitions for attack that I've seen before include "Strike" , "Assault or criticise violently" , "Offensive action" , "Brownish pigment" , "Pitch into" .)