Pleasure-seeker's complaint involving a bad back (8)
I believe the answer is:
gadabout
'pleasure-seeker's' is the definition.
(gadabout can mean a pleasure-seeker)
'complaint involving a bad back' is the wordplay.
'complaint' becomes 'gout' (gout is a kind of disease).
'involving' means one lot of letters goes inside another.
'back' shows that the letters should be reversed in order.
'bad' back-to-front is 'dab'.
'a'+'dab'='adab'
'gout' going around 'adab' is 'GADABOUT'.
(Other definitions for gadabout that I've seen before include "restless type" , "Person who wanders aimlessly in search of pleasure" , "Restless seeker after amusement" , "restless sort" , "Rover" .)