Carry on adopting Oscar’s ruse (4)
I believe the answer is:
ploy
'ruse' is the definition.
(both can mean a trick or scheme)
'carry on adopting oscar's' is the wordplay.
'carry on' becomes 'ply' (ply can mean to work continually at something).
'adopting' indicates putting letters inside (inserted letters are taken on or adopted).
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'ply' placed around 'o' is 'PLOY'.
(Other definitions for ploy that I've seen before include "Game manoeuvre" , "Cunning manoeuvre to gain advantage" , "Clever tactic" , "Device" , "Trick" .)