Disloyal? Not so! (6)
I believe the answer is:
untrue
'disloyal?' is the definition.
('untrue' can be similar in meaning to 'disloyal')
'not so' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'so' becomes 'true' (similar in meaning).
'un'+'true'='UNTRUE'
(Other definitions for untrue that I've seen before include "Spurious" , "Incorrect or false" , "Not loyal" , "Fallacious" , "Counter-factual" .)