Vegetable is cold and tough (5)
I believe the answer is:
chard
'vegetable' is the definition.
(chard is a kind of vegetable)
'cold and tough' is the wordplay.
'cold' becomes 'c' (eg on taps).
'and' means one lot of letters go next to another.
'tough' becomes 'hard' ('hard' can be a synonym of 'tough').
'c'+'hard'='CHARD'
'is' acts as a link.
(Other definitions for chard that I've seen before include "Beet with edible leaves and stalks" , "salad" , "Leaf stalks from a kind of beet" , "A beet; sounds like burnt" , "Variety of beet with edible white leaf stalks" .)