If not cloudy, take top off (6)
I believe the answer is:
unless
'if not' is the definition.
(I've seen this in another clue)
'cloudy take top off' is the wordplay.
'cloudy' becomes 'sunless' (similar in meaning).
'take top off' means to remove the first letter (I've seen 'taking top off' mean this).
'sunless' with its first letter taken off is 'UNLESS'.
(Other definitions for unless that I've seen before include "Except on the condition that" , "Though" , "Otherwise than if" , "Conditional on" , "Conjunction meaning except when" .)