Windless and cold over Harlem occasionally (4)
I believe the answer is:
calm
'windless' is the definition.
(similar in meaning)
'cold over harlem occasionally' is the wordplay.
'cold' becomes 'C' (abbreviation, eg on taps).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternating letters of 'harlem' are 'alm'.
'c'+'alm'='CALM'
'and' acts as a link.
(Other definitions for calm that I've seen before include "Unruffled, not excited" , "'After a storm comes a ....'" , "Cool, composed" , "Placid, unexcited" , "Tranquil, undisturbed" .)