Break down fences behind hyrax (6)
I believe the answer is:
dassie
'hyrax' is the definition.
(I know that hyrax can be written as dassie)
'break down fences behind' is the wordplay.
'break down' becomes 'die' (synonyms).
'fences' means one lot of letters goes inside another (inserted letters are fenced off).
'behind' becomes 'ass' (both can mean one's back side).
'die' placed around 'ass' is 'DASSIE'.
(Other definitions for dassie that I've seen before include "One animal" , "rock rabbit" , "Hyrax" .)