In his business there's much promotion (5)
I believe the answer is:
adman
'in his business there's much promotion' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'in his business there's much promotion' is the wordplay.
'in' means one lot of letters goes inside another.
'his business' becomes 'adm' (I can't justify this - if you can you should give a lot more credence to this answer).
'there's' becomes 'n' (I've seen this in other clues).
'much' means to remove the last letter (much but not all of the word).
'promotion' becomes 'ad' (short for advert).
'adm'+'n'='admn'
'ad' with its final letter taken off is 'a'.
'admn' going around 'a' is 'ADMAN'.
Can you help me to learn more?
(Other definitions for adman that I've seen before include "Commercial publicist" , "Someone involved in commercial promotion" , "Worker in commercials" , "persuasive type" , "promoter" .)