Resign from actor's union without case (4)
I believe the answer is:
quit
'resign from' is the definition.
(to quit something is to resign from it)
'actor's union without case' is the wordplay.
'actor's union' becomes 'Equity' (Equity is the trade union for UK actors).
'without case' indicates the central letters (letters which encase the word are removed).
The centre of 'equity' is 'QUIT'.
(Other definitions for quit that I've seen before include "Leave or stop doing" , "Decamp" , "Stop; abandon" , "Depart, resign from" , "Go away from or abandon task" .)