With support large Yankee becomes sly and treacherous (8)
I believe the answer is:
weaselly
'treacherous' is the definition.
(I've seen this before)
'with support large yankee' is the wordplay.
'with' becomes 'w' (abbreviation).
'support' becomes 'easel' (easel is a kind of support).
'large' becomes 'L' (eg in clothes sizes).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'w'+'easel'+'l'+'y'='WEASELLY'
'becomes sly and' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for weaselly that I've seen before include "Sly, treacherous" , "Difficult to trust" .)