Ill-gotten gain's turned up in seat (5)
I believe the answer is:
stool
'seat' is the definition.
(stool is a kind of seat)
'ill-gotten gain's turned up' is the wordplay.
'ill-gotten gain's' becomes 'loot's' (loot means stolen items).
'turned up' says the letters should be written backwards.
'loots' written backwards gives 'STOOL'.
'in' acts as a link.
(Other definitions for stool that I've seen before include "Armless seat" , "Motion" , "Backless chair" , "item of furniture here" , "Small seat" .)