Geraint turns out to be a thankless person (7)
I believe the answer is:
ingrate
'a thankless person' is the definition.
(someone who is ungrateful)
'geraint turns out' is the wordplay.
'turns out' indicates anagramming the letters.
'geraint' with letters rearranged gives 'INGRATE'.
'to be' acts as a link.
(Other definitions for ingrate that I've seen before include "Ungrateful person" , "Does one not appreciate" , "Thankless type" , "Unappreciative type" , "Unappreciative person" .)