Treacherous person with support (6)
I believe the answer is:
weasel
'treacherous person' is the definition.
(I've seen this before)
'with support' is the wordplay.
'with' becomes 'w' (abbreviation).
'support' becomes 'easel' (easel is a kind of support).
'w'+'easel'='WEASEL'
(Other definitions for weasel that I've seen before include "These animal words are intentionally misleading" , "Rat" , "Stoat-like mammal - treacherous type" , "Seal we turn into a long thin mammal" , "Predator" .)