Pigeon let down catching insect (7)
I believe the answer is:
fantail
'pigeon' is the definition.
(type of pigeon)
'let down catching insect' is the wordplay.
'let down' becomes 'fail' (failing is a kind of letting down).
'catching' is an insertion indicator.
'insect' becomes 'ant' (ant is a kind of insect).
'fail' enclosing 'ant' is 'FANTAIL'.
(Other definitions for fantail that I've seen before include "Flier" , "Breed of pigeon" , "In a flat (anag.)" , "Bird" , "Variety of pigeon" .)