If not completely, is almost without sun (6)
I believe the answer is:
unless
'if not' is the definition.
(I've seen this before)
'almost without sun' is the wordplay.
'almost' means to remove the first letter.
'without sun' becomes 'sunless'.
'sunless' with its initial letter taken away is 'UNLESS'.
'completely is' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for unless that I've seen before include "Except, save" , "Conjunction meaning except when" , "bar" , "Except on the condition that" , "Otherwise than if" .)