Area with grand river and a mausoleum site (4)
I believe the answer is:
agra
'mausoleum site' is the definition.
(the Taj Mahal is in Agra in India)
'area with grand river and a' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'with' is a charade indicator (letters next to each other).
'grand' becomes 'g' (G is short for grand as in a thousand).
'river' becomes 'r' (abbreviation for river).
'and' says to put letters next to each other.
'a'+'g'+'r'+'a'='AGRA'
(Other definitions for agra that I've seen before include "Taj Mahal city" , "Indian masterpiece" , "city founded in 1566" , "Eastern city" , "Site of the Taj Mahal" .)