Bright, but originally wet (6)
I believe the answer is:
brainy
'bright' is the definition.
(thesaurus)
'but originally wet' is the wordplay.
'originally' suggests taking the first letters.
'wet' becomes 'rainy' ('rainy' can be similar in meaning to 'wet').
The first letter of 'but' is 'b'.
'b'+'rainy'='BRAINY'
(Other definitions for brainy that I've seen before include "Binary (anag.)" , "smart?" , "Intelligent, clever." , "Highly intelligent" , "Bright" .)