Bird dog loses lead (5)
I believe the answer is:
eagle
'bird' is the definition.
(eagle is a type of bird)
'dog loses lead' is the wordplay.
'dog' becomes 'beagle' (beagle is a kind of dog).
'loses lead' means to remove the first letter (I've seen 'losing lead' mean this).
'beagle' with its first letter taken away is 'EAGLE'.
(Other definitions for eagle that I've seen before include "ten-dollar coin" , "Score of two strokes under par for a golf hole" , "born hunter" , "experts are putting more than albatross?" , "part of round that's exceptional" .)