Treacherous person initially without support (6)
I believe the answer is:
weasel
'treacherous person' is the definition.
(weasel can mean a nasty or deceitful person)
'initially without support' is the wordplay.
'initially' says to take the initial letters.
'support' becomes 'easel' (easel is a kind of support).
The first letter of 'without' is 'w'.
'w'+'easel'='WEASEL'
(Other definitions for weasel that I've seen before include "Small carnivore" , "Carnivore; treacherous person (colloq.)" , "Proverbially deceitful mammal" , "Stoat-like animal" , "Scoundrel" .)