Tub-thumper's line in business (6)
I believe the answer is:
tirade
'tub-thumper's' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'line in business' is the wordplay.
'line' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'business' becomes 'trade' (trade is a kind of business).
'i' going inside 'trade' is 'TIRADE'.
Can you help me to learn more?
(Other definitions for tirade that I've seen before include "A long abusive harangue" , "Prolonged speech of abuse" , "Long speech of angry criticism" , "Fulmination" , "Verbal onslaught" .)