Pants about popular birds (7)
I believe the answer is:
puffins
'birds' is the definition.
(puffin is a kind of bird)
'pants about popular' is the wordplay.
I cannot really understand how this works, but
'popular' could be 'in' ('in' can mean trendy) and 'in' is located in the answer.
'pants' could be 'puffs' ('puff' can be a synonym of 'pant') and 'puffs' is found within the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for puffins that I've seen before include "Seabirds with brightly-coloured bills" , "Black and white diving birds" , "Bright-billed auks" .)