So a son, true to form, is disloyal (10)
I believe the answer is:
treasonous
'disloyal' is the definition.
('treasonous' can be similar in meaning to 'disloyal')
'so a son true to form' is the wordplay.
'to form' indicates anagramming the letters.
'so'+'a'+'son'+'true'='soasontrue'
'soasontrue' anagrammed gives 'TREASONOUS'.
'is' is the link.
(Other definitions for treasonous that I've seen before include "criminal" , "liable to harm the homeland" , "disloyal" , "Betraying one's country" , "Like Benedict Arnold, for some!" .)