In a back street, it's cold! (5)
I believe the answer is:
frost
'in' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'a back street it's cold' is the wordplay.
I cannot really understand how this works, but
'back' could be 'fro' (synonyms) and 'fro' is present in the answer.
'street' could be 'st' (abbreviation) and 'st' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for frost that I've seen before include "Frozen dew" , "Icy condensation" , "Coldness of manner" , "Robert ---, US poet" , "RD Wingfield's detective" .)