Trim round notice for procession (6)
I believe the answer is:
parade
'for procession' is the definition.
(parade is a kind of procession)
'trim round notice' is the wordplay.
'trim' becomes 'pare' (synonyms).
'round' means one lot of letters goes inside another.
'notice' becomes 'ad' (both can mean an advertisement).
'pare' going around 'ad' is 'PARADE'.
(Other definitions for parade that I've seen before include "Assembly of troops for inspection" , "Show - display" , "Cavalcade or procession" , "Carnival" , "Public procession as on St Patrick's Day" .)