Fine bottom, but not much of a looker (5)
I believe the answer is:
frump
'not much of a looker' is the definition.
(term for an unattractive woman)
'fine bottom' is the wordplay.
'fine' becomes 'f' (abbreviation).
'bottom' becomes 'rump' (synonyms).
'f'+'rump'='FRUMP'
'but' acts as a link.
(Other definitions for frump that I've seen before include "Jeer" , "A dowdily-dressed unattractive woman" , "Dowdy person" , "old maid?" , "Dowdily dressed female" .)