Except if overcast, going topless (6)
I believe the answer is:
unless
'except if' is the definition.
(I've seen this before)
'overcast going topless' is the wordplay.
'overcast' becomes 'sunless' ('sunless' can be a synonym of 'overcast').
'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 "Except, save" , "Otherwise than if" , "Conditional on" , "Though" , "Conjunction meaning except when" .)