A day in Herts town doing business (7)
I believe the answer is:
trading
'doing business' is the definition.
(thesaurus)
'a day in herts town' is the wordplay.
'a day' becomes 'ad' (I've seen this in other clues**).
'in' means one lot of letters goes inside another.
'herts town' becomes 'tring' (I've seen this before).
'ad' going into 'tring' is 'TRADING'.
(Other definitions for trading that I've seen before include "Buying and selling" , "Exchanging; a word used in the first ever 2 dn" , "In business" , "Dealing" , "in the marketplace" .)