Faithless son mostly purified after conversion (10)
I believe the answer is:
perfidious
'faithless' is the definition.
(similar in meaning)
'son mostly purified after conversion' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'after conversion' indicates anagramming the letters (I've seen this in other clues).
'son' with its last letter taken away is 'so'.
'so'+'purified'='sopurified'
'sopurified' anagrammed gives 'PERFIDIOUS'.
(Other definitions for perfidious that I've seen before include "Deceitful, untrustworthy" , "Like Albion perhaps" .)