Part of Sheffield — or a don's fabulous city (2,6)
I believe the answer is:
'city' is the definition.
(I've seen this before)
'part of sheffield or a don's fabulous' is the wordplay.
'part of' says the answer is hidden in the clue.
'EL DORADO' can be found hidden inside 'sheffield or a dons fabulous'.
(Other definitions for el dorado that I've seen before include "Place of fabulous opportunity" , "Fabled place of wealth" , "Land of great wealth" , "Fabled city of great riches" , "Golden land imagined by the Spanish explorers of America" .)