Wise bird swallows start to irritate (5)
I believe the answer is:
ernie
'wise' is the definition.
(comedian Ernie Wise)
'bird swallows start to irritate' is the wordplay.
'bird' becomes 'erne' (erne is a kind of bird).
'swallows' means one lot of letters goes inside another.
'start to' says to take the initial letters.
The first letter of 'irritate' is 'i'.
'erne' enclosing 'i' is 'ERNIE'.
(Other definitions for ernie that I've seen before include "Male" , "Man - chooses Premium Bond winners" , "- (the Fastest Milkman in the West) (Benny Hill)" , "Boy" , "Premium Bond picker" .)