If not overcast, taking top off (6)
I believe the answer is:
unless
'if not' is the definition.
(I've seen this before)
'overcast taking top off' is the wordplay.
'overcast' becomes 'sunless' (synonyms).
'taking top off' 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 "Except, save" , "Though" , "Otherwise than if" , "Except on the condition that" , "Conditional on" .)