Young woman's large bottom (4)
I believe the answer is:
lass
'young woman's' is the definition.
(lass is a kind of young woman)
'large bottom' is the wordplay.
'large' becomes 'l'.
'bottom' becomes 'ass' ('ass' can be a synonym of 'bottom'**).
'l'+'ass'='LASS'
(Other definitions for lass that I've seen before include "Young woman (-. Eng.)" , "Girl (of Richmond Hill?)" , "Familiar form of address for female" , "Damsel" , "Sweet resident of Richmond Hill?" .)