Target of abuse, relative's friend (4,5)
I believe the answer is:
aunt sally
'target of abuse' is the definition.
(I've seen this before)
'relative's friend' is the wordplay.
'relative's' becomes 'aunts' (aunt is a kind of relative).
'friend' becomes 'ally' ('ally' can be a synonym of 'friend').
'aunts'+'ally'='AUNT SALLY'
(Other definitions for aunt sally that I've seen before include "One easily attacked" , "Female relation used as a target in throwing game" , "Target for abuse" , "object of ridicule" , "Target of general criticism, as in throwing game" .)