Vegetable that's cold and tough (5)
I believe the answer is:
chard
'vegetable' is the definition.
(I know that chard is a type 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' (synonyms).
'c'+'hard'='CHARD'
'that's' is the link.
(Other definitions for chard that I've seen before include "vegetable crop" , "Kind of (Swiss?) beet with edible leaves" , "salad" , "Seakale beet" , "Variety of beet with edible leaves" .)