If they're blue, that's fine (5)
I believe the answer is:
skies
'if' is the definition.
I can't tell whether this defines the answer.
'blue that's fine' is the wordplay.
I cannot really see how this works, but
'that's' could be 'ie' (abbreviation for 'id est', 'that is' in Latin) and 'ie' is located in the answer.
This explanation may well be incorrect...
'they're' acts as a link.
Can you help me to learn more?
(Other definitions for skies that I've seen before include "Heavens" , "Hits high in the air" .)