Inundate exchange, taking in million (5)
I believe the answer is:
swamp
'inundate' is the definition.
('swamp' can be a synonym of 'inundate')
'exchange taking in million' is the wordplay.
'exchange' becomes 'swap' (swap is a kind of exchange).
'taking in' is an insertion indicator.
'million' becomes 'm' (abbreviation).
'swap' going around 'm' is 'SWAMP'.
(Other definitions for swamp that I've seen before include "Boggy ground" , "Bog, marsh" , "Seasonally-flooded lowland" , "In flood" , "Marshy land" .)