Nasty red-backed snake (7)
I believe the answer is:
meander
'snake' is the definition.
(thesaurus)
'nasty red-backed' is the wordplay.
'nasty' becomes 'mean' ('mean' can be similar in meaning to 'nasty'**).
'backed' says the letters should be written in reverse.
'red' back-to-front is 'der'.
'mean'+'der'='MEANDER'
(Other definitions for meander that I've seen before include "Dear men (anag) - wander" , "Wander aimlessly like a stream" , "circuitous route" , "Indirectly move" , "Wandering direction" .)