Sharp tail of garden bird (7)
I believe the answer is:
bittern
'bird' is the definition.
(bittern is a kind of bird)
'sharp tail of garden' is the wordplay.
'sharp' becomes 'bitter' (I've seen this before).
'tail of' indicates one should take the final letters.
The last letter of 'garden' is 'n'.
'bitter'+'n'='BITTERN'
(Other definitions for bittern that I've seen before include "Marshland bird" , "Wading bird" , "Compact tawny-brown heron with nocturnal habits and a booming cry" , "Aquatic bird" , "Marsh bird of the heron family" .)