Doing business, placing advert in Herts town (7)
I believe the answer is:
trading
'doing business' is the definition.
(I've seen this before)
'advert in herts town' is the wordplay.
'advert' becomes 'ad' (synonyms).
'in' indicates putting letters inside.
'herts town' becomes 't ring' (I've seen this before).
'ad' put within 'tring' is 'TRADING'.
'placing' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for trading that I've seen before include "Exchanging; a word used in the first ever 2 dn" , "Dealing" , "In business" , "Commercial activity" , "in the marketplace" .)