Old Italian’s endless faith in English jail (8)
I believe the answer is:
etruscan
'old italian's' is the definition.
'endless faith in english jail' is the wordplay.
'endless' means to remove the last letter.
'faith' becomes 'trust' ('trust' can be a synonym of 'faith').
'in' is an insertion indicator.
'english' becomes 'e' (abbreviation**).
'jail' becomes 'can' (the can is American slang for prison).
'trust' with its last letter taken away is 'trus'.
'e'+'can'='ecan'
'trus' inserted inside 'ecan' is 'ETRUSCAN'.
(Other definitions for etruscan that I've seen before include "Of an ancient people of central Italy" , "from part of old Italy" , "Sure can't refer to ancient state in Italy" , "Ancient language of W. Italy" , "Centaurs (anag) -- native of an ancient state of central Italy" .)