Bird finally landing, breathing heavily (7)
I believe the answer is:
puffing
'breathing heavily' is the definition.
(I've seen this before)
'bird finally landing' is the wordplay.
'bird' becomes 'puffin' (puffin is a kind of bird).
'finally' indicates one should take the final letters.
The last letter of 'landing' is 'g'.
'puffin'+'g'='PUFFING'
(Other definitions for puffing that I've seen before include "gasping for breath" , "Obviously unfit" , "Breathing heavily" , "Breathing noisily" , "advertising" .)