Seaside location, sunny and happening (8)
I believe the answer is:
Here is my best explanation:
'seaside location' is the definition.
'sunny and happening' is the subsidiary indication.
'sunny' becomes 'bright' ('bright' can be similar in meaning to 'sunny').
'and' is a charade indicator (letters next to each other).
'happening' becomes 'on'.
(Other definitions for brighton that I've seen before include "East Sussex resort", "South coast city combined with Hove", "Sussex city (with Hove)", "Graham Greene wrote ` `........ Rock' '", "Where the Prince Regent built his Pavilion", "South-coast resort", "English resort with big thorn", "Sussex seaside resort", "South coast resort" and "Sussex coastal resort".)