Harry taken in by smooth cheat (7)
I believe the answer is:
finagle
'cheat' is the definition.
(I've seen this before)
'harry taken in by smooth' is the wordplay.
'harry' becomes 'nag' (both can mean to consistently bother).
'taken in by' indicates putting letters inside.
'smooth' becomes 'file' (filing is a kind of smoothing).
'nag' going within 'file' is 'FINAGLE'.
(Other definitions for finagle that I've seen before include "Swindle" , "cheat" , "Dishonestly obtain" .)