No longer cold when one goes out (4)
I believe the answer is:
once
'no longer' is the definition.
(I've seen this in another clue)
'cold when one goes out' is the wordplay.
I cannot really understand how this works, but
'cold' could be 'c' (eg on taps) and 'c' is located in the answer.
'when' could be 'on' (I've seen this in other clues) and 'on' is found within the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for once that I've seen before include "At some time past" , "'... bitten, twice shy'" , "In days past" , "Some time back" , "Previously" .)