In the City, finally leave union (6)
I believe the answer is:
equity
'union' is the definition.
(I've seen this before)
'in the city finally leave' is the wordplay.
'in' indicates putting letters inside.
'finally' suggests the final letters.
'leave' becomes 'quit' (quitting is a kind of leaving).
The last letters of 'the city' are 'ey'.
'ey' enclosing 'quit' is 'EQUITY'.
(Other definitions for equity that I've seen before include "actors' group" , "Fair play - actors' union" , "Even-handedness" , "Fairness for actors' trade union" , "Moral justice for the actor's union" .)