Short of breath, bird tiring in the end (7)
I believe the answer is:
puffing
'short of breath' is the definition.
'bird tiring in the end' is the wordplay.
'bird' becomes 'puffin' (puffin is a kind of bird).
'in the end' suggests the final letters.
The final letter of 'tiring' is 'g'.
'puffin'+'g'='PUFFING'
(Other definitions for puffing that I've seen before include "Breathing heavily" , "gasping for breath" , "Breathing noisily" , "advertising" , "Obviously unfit" .)