Mark and Pansy turned up for a biscuit (6,4)
I believe the answer is:
brandy snap
'biscuit' is the definition.
(type of biscuit)
'mark and pansy turned up' is the wordplay.
'mark' becomes 'brand' (synonyms).
'and' means one lot of letters go next to another.
'turned up' says the letters should be written backwards.
'pansy' written backwards gives 'ysnap'.
'brand'+'ysnap'='BRANDY-SNAP'
'for a' acts as a link.
(Other definitions for brandy snap that I've seen before include "Cookie" , "Crisp confection" , "Cream-filled gingerbread wafer" , "Crisp ginger biscuit" , "Crisp, rolled biscuit" .)