Like toffee cold: cut an unknown quantity (5)
I believe the answer is:
chewy
'like toffee' is the definition.
(I've seen this before)
'cold cut an unknown quantity' is the wordplay.
'cold' becomes 'c' (eg on taps).
'cut' becomes 'hew' (I've seen this before).
'an unknown quantity' becomes 'y' (algebra).
'c'+'hew'+'y'='CHEWY'
(Other definitions for chewy that I've seen before include "Taking much mastication" , "Soft and sticky (to eat)" , "With a texture like toffee" , "Tough to masticate" , "Requiring mastication, like toffee say" .)