Economic extremes explode with chest (4,3,4)
I believe the answer is:
boom and bust
'economic extremes' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'explode with chest' is the wordplay.
'explode' becomes 'boom' (I can't justify this - if you can you should believe this answer much more).
'with' becomes 'and'.
'chest' becomes 'bust' (bust can mean a woman's chest).
'boom'+'and'+'bust'='BOOM-AND-BUST'
Can you help me to learn more?
(Other definitions for boom and bust that I've seen before include "buoyant in waves?" , "Change of fortune" , "see-saw with economy" , "Cycle with economy" .)