Except if it's cloudy, no start (6)
I believe the answer is:
unless
'except if' is the definition.
('unless' can mean 'except if...')
'it's cloudy no start' is the wordplay.
I cannot quite see how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is present in the answer.
'no' could be 'n' (common abbreviation) and 'n' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for unless that I've seen before include "Except, save" , "Except on the condition that" , "Conditional on" , "bar" , "Otherwise than if" .)