He can't be trusted with artist's support (6)
I believe the answer is:
Here is my best explanation:
'he can't be trusted' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'with artist's support' is the subsidiary indication.
'with' becomes 'w' (abbreviation).
'artist's support' becomes 'easel' (I've seen this before).
(Other definitions for weasel that I've seen before include "Small fierce animal with slender body and reddish fur", "Animal; sly person", "Stoat-like mammal - treacherous type", "Animal related to the stoat", "E Wales (anag.)", "Cowardly carnivore", "Boggle", "Slender animal", "Equivocate - small animal" and "Proverbially deceitful mammal".)