Vegetable slice mostly cold and brown (8)
I believe the answer is:
cucumber
'vegetable' is the definition.
(cucumber is a kind of vegetable)
'slice mostly cold and brown' is the wordplay.
'slice' becomes 'cut' ('cut' can be a synonym of 'slice').
'mostly' means to remove the last letter.
'cold' becomes 'c' (eg on taps).
'and' says to put letters next to each other.
'brown' becomes 'umber' (umber is a kind of brown**).
'cut' with its final letter removed is 'cu'.
'cu'+'c'+'umber'='CUCUMBER'
(Other definitions for cucumber that I've seen before include "Proverbially cool salad plant" , "Green salad fruit" , "Long salad vegetable" , "Three dn item" , "Ecu crumb (anag.)" .)