Except when cloudy, going topless (6)
I believe the answer is:
unless
'except when' is the definition.
(I've seen this in another clue)
'cloudy going topless' is the wordplay.
'cloudy' becomes 'sunless' (similar in meaning).
'going topless' means to remove the first letter.
'sunless' with its initial letter taken away is 'UNLESS'.
(Other definitions for unless that I've seen before include "Conditional on" , "Otherwise than if" , "Conjunction meaning except when" , "Though" , "bar" .)