Engage half of them on stratagem (6)
I believe the answer is:
employ
'engage' is the definition.
('employ' can be a synonym of 'engage')
'half of them on stratagem' is the wordplay.
'half of' suggests halving the letters.
'on' says to put letters next to each other.
'stratagem' becomes 'ploy' ('ploy' can be a synonym of 'stratagem').
'them' with half the letters taken is 'em'.
'em'+'ploy'='EMPLOY'
(Other definitions for employ that I've seen before include "Give work to (someone)" , "Use or take on" , "Take into service" , "Utilise" , "Draw on" .)