Mum’s sister’s friend is an object of criticism (4,5)
I believe the answer is:
aunt sally
'object of criticism' is the definition.
'aunt sally' can be an answer for 'object' (I have seen 'object of ridicule' mean 'aunt sally' so perhaps 'object' could also mean 'aunt sally'). I'm not sure about the 'of criticism' bit.
'mum's sister's friend is an' is the wordplay.
I cannot quite understand how this works, but
'friend' could be 'ally' (an ally is a friend or supporter) and 'ally' is found in the answer.
'an' could be 'a' and 'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for aunt sally that I've seen before include "Deliberately easy target for criticism" , "Target of general criticism, as in throwing game" , "Family friend" , "She's a target in throwing games, like female relative" , "scapegoat" .)