Drunk abstains until becoming weak (13)
I believe the answer is:
insubstantial
'weak' is the definition.
(similar in meaning)
'drunk abstains until' is the wordplay.
'drunk' indicates anagramming the letters.
'abstains'+'until'='abstainsuntil'
'abstainsuntil' anagrammed gives 'INSUBSTANTIAL'.
'becoming' acts as a link.
(Other definitions for insubstantial that I've seen before include "Meagre" , "Flimsy" , "Lacking solidity" , "Tenuous" .)