Left a fleet being organised for an advertisement (7)
I believe the answer is:
leaflet
'for an advertisement' is the definition.
(thesaurus)
'left a fleet being organised' is the wordplay.
'left' becomes 'l' (common abbreviation).
'being organised' indicates an anagram.
'a'+'fleet'='afleet'
'afleet' is an anagram of 'eaflet'.
'l'+'eaflet'='LEAFLET'
(Other definitions for leaflet that I've seen before include "Sheet of paper giving information" , "Printed sheet, flyer" , "very little greenery" , "Handbill, flyer" , "It may advertise" .)