Flap on underwear: popular idea (9)
I believe the answer is:
brainwave
'idea' is the definition.
(I've seen this before)
'flap on underwear popular' is the wordplay.
'flap' becomes 'wave' (synonyms).
'on' is a charade indicator (letters next to each other).
'underwear' becomes 'bra' (bra is a type of undergarment).
'popular' becomes 'in' (something that's 'in' is fashionable).
'bra'+'in'='brain'
'wave' after 'brain' is 'BRAINWAVE'.
(Other definitions for brainwave that I've seen before include "Good idea" , "Sudden bright idea" , "Great idea" .)