Rant in exchange about onset of inflation (6)
I believe the answer is:
tirade
'rant' is the definition.
(thesaurus)
'exchange about onset of inflation' is the wordplay.
'exchange' becomes 'trade' (trade is a kind of exchange).
'about' means one lot of letters goes inside another.
'onset of' indicates taking the first letters.
The initial letter of 'inflation' is 'i'.
'trade' going around 'i' is 'TIRADE'.
'in' is the link.
(Other definitions for tirade that I've seen before include "A long, vehement harangue" , "A prolonged outburst of angry words" , "Sustained abuse" , "Long, abusive speech" , "Verbal outburst" .)