Bit of sunshine outside in wet (5)
I believe the answer is:
rainy
'wet' is the definition.
(similar in meaning)
'bit of sunshine outside in' is the wordplay.
'bit of sunshine' becomes 'ray' (I've seen this before).
'outside' means one lot of letters goes inside another.
'ray' going around 'in' is 'RAINY'.
(Other definitions for rainy that I've seen before include "(Of weather) wet" , "With water falling outside" , "Drizzly" , "Wet, showery" , "The day one saves for" .)