From the biting cold, help to protect (4)
I believe the answer is:
acid
'from the biting' is the definition.
I can't judge whether this definition defines the answer.
'cold help to protect' is the wordplay.
'cold' becomes 'C' (eg on taps).
'help' becomes 'aid' (synonyms).
'to protect' is an insertion indicator (inserted letters are held or protected).
'c' put into 'aid' is 'ACID'.
Can you help me to learn more?
(Other definitions for acid that I've seen before include "Sour to the taste" , "Vinegary" , "Corrosive fluid" , "Corrosive liquid; drug" , "Alkali's opposite" .)