Seat for sort of pigeon (5)
I believe the answer is:
stool
I believe this clue is a double definition.
'seat' is the first definition.
(stool is a kind of seat)
'sort of pigeon' is the second definition.
(a stool pigeon is a police informer)
'for' acts as a link.
(Other definitions for stool that I've seen before include "Motion" , "Seat without a back" , "place butt on one?" , "Armless seat" , "Three-legged backless seat" .)