Trick, good during last act (7)
I believe the answer is:
finagle
'trick' is the definition.
(to finagle something is to get it through trickery)
'good during last act' is the wordplay.
'good' becomes 'g' (abbreviation).
'during' is an insertion indicator.
'last act' becomes 'finale' (I've seen this before).
'g' put within 'finale' is 'FINAGLE'.
(Other definitions for finagle that I've seen before include "Dishonestly obtain" , "Swindle" , "cheat" .)