Faithful groom hit Canute (9)
I believe the answer is:
authentic
'faithful' is the definition.
(synonyms)
'groom hit canute' is the wordplay.
'groom' indicates anagramming the letters.
'hit'+'canute'='hitcanute'
'hitcanute' with letters rearranged gives 'AUTHENTIC'.
(Other definitions for authentic that I've seen before include "24 [LEGIT]" , "reliable" , "Genuine, not a copy" , "Real, genuine" , "Ian, the cut is genuine" .)