Unearth a marsupial seller (4,3)
I believe the answer is:
root out
'unearth' is the definition.
(thesaurus)
'a marsupial seller' is the wordplay.
'a marsupial' becomes 'roo' (short for kangaroo).
'seller' becomes 'tout' (tout is a kind of seller).
'roo'+'tout'='ROOT OUT'
(Other definitions for root out that I've seen before include "Extirpate, eradicate" , "Extirpate, find and destroy" , "Destroy completely" , "Completely remove" , "Get rid or, extirpate" .)