Popular monarch stopping street advertising (6)
I believe the answer is:
insert
'advertising' is the definition.
(advertisement inserted into a publication)
'popular monarch stopping street' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'stopping' indicates putting letters inside (stopping can mean blocking a gap).
'street' becomes 'st' (abbreviation).
'er' inserted within 'st' is 'sert'.
'in'+'sert'='INSERT'
(Other definitions for insert that I've seen before include "Put thing in between others" , "Put between or within other things" , "Place within, pages of a publication say" , "supplement" , "Place (something) inside (something else)" .)