Mum's fish and single cheese (10)
I believe the answer is:
mascarpone
'cheese' is the definition.
(I know that mascarpone is a type of cream cheese)
'mum's fish and single' is the wordplay.
'mum's' becomes 'ma's' (ma can mean mother).
'fish' becomes 'carp' (carp is a kind of fish).
'and' means one lot of letters go next to another.
'single' becomes 'one' (a single thing - only one).
'mas'+'carp'+'one'='MASCARPONE'
(Other definitions for mascarpone that I've seen before include "Soft Italian cheese" , "Italian cream cheese" .)