Turn nasty before claret served up (7)
I believe the answer is:
meander
'turn' is the definition.
(thesaurus)
'nasty before claret served up' is the wordplay.
'nasty' becomes 'mean' (similar in meaning).
'before' is a charade indicator (letters next to each other).
'claret' becomes 'red' (red wine).
'served up' shows that the letters should be reversed in order (in a down clue, going upwards).
'red' reversed gives 'der'.
'mean'+'der'='MEANDER'
(Other definitions for meander that I've seen before include "Indirectly move" , "Aimless course" , "Follow a random path" , "an Indo-European river" , "Dear men (anag) - wander" .)