As a genial drunk, feeling no pain (9)
I believe the answer is:
analgesia
'feeling no pain' is the definition.
(analgesia is a painlessness)
'as a genial drunk' is the wordplay.
'drunk' indicates anagramming the letters.
'as'+'a'+'genial'='asagenial'
'asagenial' with letters rearranged gives 'ANALGESIA'.
(Other definitions for analgesia that I've seen before include "As a linage (anag) - insensibility to pain" , "Absence of pain" , "Inability to feel pain" , "painless" , "Loss of pain" .)