Bird that hurts gerbil’s tail (3)
I believe the answer is:
owl
'bird' is the definition.
(owl is a kind of bird)
'that hurts gerbil's tail' is the wordplay.
'that hurts' becomes 'ow'.
'tail' says to take the final letters.
The last letter of 'gerbil' is 'l'.
'ow'+'l'='OWL'
(Other definitions for owl that I've seen before include "Wise old bird" , "Traditionally wise bird" , "Smuggle sheep" , "Nightbird" , "Bird of the 18" .)