Substantial part of agricultural Argentina (5)
I believe the answer is:
large
'substantial' is the definition.
(I've seen this before)
'part of agricultural argentina' is the wordplay.
'part of' indicates a hidden word.
'LARGE' is hidden in the letters of 'agricultural argentina'.
(Other definitions for large that I've seen before include "Of great size" , "Titanic" , "Little's late partner" , "Elgar (anag)" , "Vast" .)