If not overcast, going topless (6)
I believe the answer is:
unless
'if not' is the definition.
(I've seen this before)
'overcast going topless' is the wordplay.
'overcast' becomes 'sunless' (synonyms).
'going topless' means to remove the first letter.
'sunless' with its first letter taken away is 'UNLESS'.
(Other definitions for unless that I've seen before include "Otherwise than if" , "Except on the condition that" , "Except, save" , "bar" , "Conjunction meaning except when" .)