Wet academic having removed cap (5)
I believe the answer is:
rainy
'wet' is the definition.
(similar in meaning)
'academic having removed cap' is the wordplay.
'academic' becomes 'brainy' (academic can mean clever or brainy).
'having removed cap' means to remove the first letter (in a down clue, cap can mean the top letter).
'brainy' with its first letter taken away is 'RAINY'.
(Other definitions for rainy that I've seen before include "Drizzly" , "(Of weather) wet" , "Save your money for this damp day" , "Save your money for a . . . . . day - to buy an umbrella?" , "The day one saves for" .)