In between sunshine, it's wet (5)
I believe the answer is:
'it's wet' is the definition.
(similar in meaning)
'in between sunshine' is the wordplay.
'between' indicates putting letters inside.
'sunshine' becomes 'ray' (I can't justify this - if you can you should believe this answer much more).
'in' going into 'ray' is 'RAINY'.
Can you help me to learn more?
(Other definitions for rainy that I've seen before include "(Of weather) wet" , "Drizzly" , "The day one saves for" , "With water falling outside" , "Save your money for this damp day" .)