Marketplace contains some performing monkey (8)
I believe the answer is:
marmoset
'monkey' is the definition.
(marmoset is a kind of monkey)
'marketplace contains some performing' is the wordplay.
'marketplace' becomes 'mart' ('mart' can be a synonym of 'marketplace').
'contains' means one lot of letters goes inside another.
'performing' indicates an anagram.
'some' anagrammed gives 'mose'.
'mart' enclosing 'mose' is 'MARMOSET'.
(Other definitions for marmoset that I've seen before include "Arms me to (anag) - animal" , "rather small creature" , "Small New World monkey" , "Somme rat? (anag)" , "Small South American monkey" .)