Nasty round scarlet snake (7)
I believe the answer is:
meander
'snake' is the definition.
(I've seen this before)
'nasty round scarlet' is the wordplay.
'nasty' becomes 'mean' (similar in meaning).
'round' says the letters should be written backwards.
'scarlet' becomes 'red' (synonyms).
'red' in reverse letter order is 'der'.
'mean'+'der'='MEANDER'
(Other definitions for meander that I've seen before include "Zigzag" , "Renamed in order to wander in leisurely way" , "Wander aimless" , "Roam aimlessly around" , "Wander aimlessly like a stream" .)