Drunk with the drunk's back trouble (7)
I believe the answer is:
tosspot
'drunk' is the definition.
(I've seen this before)
'with the drunk's back trouble' is the wordplay.
'with' says to put letters next to each other.
'the drunk' becomes 'sot' (both can mean someone who drinks too much).
'back' says the letters should be written backwards.
'trouble' becomes 'spot' (spot is a kind of trouble).
'sot' reversed gives 'tos'.
'tos' put next to 'spot' is 'TOSSPOT'.
(Other definitions for tosspot that I've seen before include "Habitual drinker" , "Fool" , "Drunk (informal)" , "supper, habitually" , "One drunkard" .)