Mock topless relative (4)
I believe the answer is:
aunt
'relative' is the definition.
(aunt is a kind of relative)
'mock topless' is the wordplay.
'mock' becomes 'taunt' (taunting is a kind of mocking).
'topless' means to remove the first letter.
'taunt' with its initial letter taken away is 'AUNT'.
(Other definitions for aunt that I've seen before include "Tuna gone bad for the relative" , "Charley's in play?" , "Put up with" , "Parent's female sibling" , "Family member" .)