Bright start in Brighton, then wet (6)
I believe the answer is:
brainy
'bright' is the definition.
(thesaurus)
'start in brighton then wet' is the wordplay.
'start in' says to take the initial letters.
'then' means one lot of letters go next to another.
'wet' becomes 'rainy' ('rainy' can be similar in meaning to 'wet').
The initial letter of 'brighton' is 'b'.
'b'+'rainy'='BRAINY'
(Other definitions for brainy that I've seen before include "Binary (anag.)" , "smart?" , "Having a high IQ" , "Intellectual" , "Bright" .)