Dowdy woman's fine bottom (5)
I believe the answer is:
frump
'dowdy woman's' is the definition.
(I've seen this before)
'fine bottom' is the wordplay.
'fine' becomes 'f' (abbreviation).
'bottom' becomes 'rump' ('rump' can be a synonym of 'bottom'**).
'f'+'rump'='FRUMP'
(Other definitions for frump that I've seen before include "Dowdy type" , "A dowdily-dressed unattractive woman" , "Dowdy person" , "Jeer" , "Dowdy-clothed woman" .)