Priam transformed into vile, heartless monster (7)
I believe the answer is:
vampire
'monster' is the definition.
(thesaurus)
'priam transformed into vile heartless' is the wordplay.
'transformed' indicates anagramming the letters.
'into' is an insertion indicator.
'heartless' suggests removing the centre.
'vile' with its centre taken out is 've'.
'priam' with letters rearranged gives 'ampir'.
'ampir' inserted within 've' is 'VAMPIRE'.
(Other definitions for vampire that I've seen before include "eg Dracula" , "Bloodsucking creature" , "Corpse that rises at night to drink the blood of the living" , "Blood-sucker (bat)" , "Transylvanian bloodsucker" .)