Millions offered in exchange are overwhelming (5)
I believe the answer is:
swamp
'are overwhelming' is the definition.
I can't judge whether this definition defines the answer.
'millions offered in exchange' is the wordplay.
'millions' becomes 'm' (abbreviation).
'offered in' indicates putting letters inside.
'exchange' becomes 'swap' (swap is a kind of exchange).
'm' put inside 'swap' is 'SWAMP'.
Can you help me to learn more?
(Other definitions for swamp that I've seen before include "Seasonally flooded low land" , "Engulf" , "Waterlogged area" , "In flood" , "Overwhelm; wetland" .)