Faith is one girl I fancy (8)
I believe the answer is:
religion
'faith' is the definition.
('religion' can be a synonym of 'faith')
'one girl i fancy' is the wordplay.
'fancy' indicates anagramming the letters.
'one'+'girl'+'i'='onegirli'
'onegirli' with letters rearranged gives 'RELIGION'.
'is' is the link.
(Other definitions for religion that I've seen before include "System of belief or worship" , "Marx's pium of the people" , "Belief - way of life of monks and nuns" , "Institution to express belief in a god" , "Particular system of belief" .)