Birds must be popular wearing pants (7)
I believe the answer is:
puffins
'birds must be' is the definition.
'puffins' can be an answer for 'birds' (I've seen this before). I am not certain of the 'must be' bit.
'popular wearing pants' is the wordplay.
I cannot quite see how this works, but
'popular' could be 'in' and 'in' is found within 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 may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for puffins that I've seen before include "Bright-billed auks" , "Seabirds with brightly-coloured bills" , "Black and white diving birds" .)