Lettuce, cold and very large (3)
I believe the answer is:
cos
'lettuce' is the definition.
(cos lettuce is a kind of lettuce)
'cold and very large' is the wordplay.
'cold' becomes 'C' (abbreviation, eg on taps).
'and' says to put letters next to each other.
'very large' becomes 'os' (abbreviation for oversize).
'c'+'os'='COS'
(Other definitions for cos that I've seen before include "Salad ingredient" , "Mozart fidelity-test opera" , "A long-leaved lettuce" , "Variety of lettuce" , "Casual as" .)