Lettuce is so cold when turned over (3)
I believe the answer is:
cos
'lettuce' is the definition.
(cos is a kind of lettuce)
'so cold when turned over' is the wordplay.
'cold when' becomes 'c' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'turned over' shows that the letters should be reversed in order.
'so'+'c'='soc'
'soc' written backwards gives 'COS'.
'is' is the link.
Can you help me to learn more?
(Other definitions for cos that I've seen before include "Variety of lettuce" , "naturally having longer leaves than most" , "Islanders here" , "something of a salad" , "leaves > function" .)