Insults arising, commercial office on top of it — something more to say? (8)
I believe the answer is:
addendum
'something more to say?' is the definition.
(I've seen this before)
'insults arising commercial office on top of it' is the wordplay.
I cannot really see how this works, but
'commercial' could be 'ad' (abbreviation for advertisement) and 'ad' is found within the answer.
'office' could be 'den' (I've seen this before) and 'den' is found within the answer.
The remaining letters 'dum' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for addendum that I've seen before include "Item appended to book" , "Dud ad-men dealt with in end of book" , "Supplement to a book" , "Thing to be added" , "Additional item at the end of a book" .)