Imagine mum with new lover (5,3)
I believe the answer is:
fancy man
'lover' is the definition.
(I know that fancy man is a type of lover)
'imagine mum with new' is the wordplay.
'imagine' becomes 'fancy' (fancying is a kind of imagining).
'mum' becomes 'ma' (ma can mean mother).
'with' is a charade indicator (letters next to each other).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'fancy'+'ma'+'n'='FANCY MAN'
(Other definitions for fancy man that I've seen before include "Slang for woman's lover - imaginative chap" , "Woman's lover (derogatory)" , "Woman's lover (colloq.)" .)