Harangue one in business (6)
I believe the answer is:
tirade
'harangue' is the definition.
(I've seen this before)
'one in business' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'business' becomes 'trade' (trade is a kind of business).
'i' placed inside 'trade' is 'TIRADE'.
(Other definitions for tirade that I've seen before include "A tongue-lashing" , "Long, abusive speech" , "Stream of invective" , "Broadside of denunciation" , "Erode or melt (material)" .)