A bird you seldom will catch (5)
I believe the answer is:
ousel
'a bird' is the definition.
(ousel is a kind of bird)
'you seldom will catch' is the wordplay.
'catch' indicates a hidden word (I've seen 'catches' mean this (the answer is caught in the clue)).
'OUSEL' is hidden in the letters of 'you seldom will'.
(Other definitions for ousel that I've seen before include "Louse turned out to be a thrush (5)." , "it goes flying" , "The bird like a blackbird from Seoul" , "Ring bird" , "Singer" .)