Mr Fitzgerald eating an ounce South Africa cheese (10)
I believe the answer is:
mozzarella
'cheese' is the definition.
(mozzarella is a kind of cheese)
'mr fitzgerald eating an ounce south africa' is the wordplay.
'fitzgerald' becomes 'ella' (I've seen this before).
'eating' means one lot of letters goes inside another.
'an ounce' becomes 'oz'.
'south africa' becomes 'za' (abbreviation for Zuid-Afrika).
'mr'+'ella'='mrella'
'oz'+'za'='ozza'
'mrella' going around 'ozza' is 'MOZZARELLA'.
(Other definitions for mozzarella that I've seen before include "Buffalo milk cheese" , "Dairy product, originally from buffaloes" , "White Italian cheese" .)