Book on trendy South Coast resort (8)
I believe the answer is:
brighton
'south coast resort' is the definition.
(I've seen this before)
'book on trendy' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'trendy' becomes 'right-on' (both can mean fashionably progressive).
'b'+'righton'='BRIGHTON'
(Other definitions for brighton that I've seen before include "South-coast resort" , "Sussex coastal resort" , "Sussex city (with Hove)" , "... hard cylindrical sweet from the seaside" , "Where the Prince Regent built his Pavilion" .)