Gasp over popular bird (6)
I believe the answer is:
puffin
'bird' is the definition.
(puffin is a kind of bird)
'gasp over popular' is the wordplay.
'gasp' becomes 'puff' ('puff' can be a synonym of 'gasp').
'over' means one lot of letters go next to another.
'popular' becomes 'in'.
'puff'+'in'='PUFFIN'
(Other definitions for puffin that I've seen before include "Species of auk" , "Seabird with a colourful bill" , "Bright-billed auk" , "Diving bird" , "Seabird (breathin' heavily?)" .)