Old man with failing clarity is very drunk (9)
I believe the answer is:
paralytic
'very drunk' is the definition.
(I've seen this before)
'old man with failing clarity' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'with' says to put letters next to each other.
'failing' indicates anagramming the letters (a failed spelling of the word).
'clarity' anagrammed gives 'ralytic'.
'pa'+'ralytic'='PARALYTIC'
'is' acts as a link.
(Other definitions for paralytic that I've seen before include "Drunk (informal)" , "Still" , "Extremely drunk" .)