Hairstyle swept up into small lump (3)
I believe the answer is:
nub
'small lump' is the definition.
'hairstyle swept up' is the wordplay.
'hairstyle' becomes 'bun'.
'swept up' shows that the letters should be reversed in order (in a down clue, letters go up).
'bun' reversed gives 'NUB'.
'into' acts as a link.
(Other definitions for nub that I've seen before include "Metal piece" , "Centre, heart" , "Central point of bun" , "Crux, central point" , "Gist; small lump" .)