One girl I converted to the faith (8)
I believe the answer is:
religion
'the faith' is the definition.
('religion' can be a synonym of 'faith')
'one girl i converted' is the wordplay.
'converted' is an anagram indicator.
'one'+'girl'+'i'='onegirli'
'onegirli' is an anagram of 'RELIGION'.
'to' is the link.
(Other definitions for religion that I've seen before include "System of divine worship" , "Organised system of faith" , "Particular system of belief" , "System of belief or worship" , "Marx's pium of the people" .)