Hoping painkiller's good (8)
I believe the answer is:
aspiring
'hoping' is the definition.
(I've seen this before)
'painkiller's good' is the wordplay.
'painkiller' becomes 'aspirin' (aspirin is a kind of painkiller).
'good' becomes 'g' (abbreviation).
'aspirin'+'g'='ASPIRING'
(Other definitions for aspiring that I've seen before include "Would-be" , "Hoping" , "Striving" , "Having ambition to achieve greater things" , "Wanting to get high" .)