Unbeliever ends in penitent suffering getting thinner (10)
I believe the answer is:
turpentine
'thinner' is the definition.
(thesaurus)
'unbeliever ends in penitent suffering' is the wordplay.
'ends' means to remove the middle letters.
'in' means one lot of letters goes inside another.
'suffering' indicates an anagram.
'unbeliever' with its middle removed is 'ur'.
'penitent' with letters rearranged gives 'tpentine'.
'ur' placed into 'tpentine' is 'TURPENTINE'.
'getting' is the link.
(Other definitions for turpentine that I've seen before include "Thick, oily resin used as paint thinner" , "Solvent" , "artist's stuff" , "Oil used in mixing paints" , "Oil used in varnishes" .)