Work round old trick (4)
I believe the answer is:
ploy
'trick' is the definition.
(I've seen this before)
'work round old' is the wordplay.
'work' becomes 'ply' (I've seen this before**).
'round' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ply' placed around 'o' is 'PLOY'.
(Other definitions for ploy that I've seen before include "Trick" , "Ruse, stratagem" , "Ruse or gimmick" , "Cunning tactic" , "Clever tactic" .)