Notice change coming across old queen's foe (9)
I believe the answer is:
adversary
'foe' is the definition.
(I've seen this in another clue)
'notice change coming across old queen's' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'change' becomes 'vary' (synonyms).
'coming across' indicates putting letters inside.
'old queen's' becomes 'ER's' (Elizabeth Regina, ie Queen Elizabeth).
'vary' enclosing 'ers' is 'versary'.
'ad'+'versary'='ADVERSARY'
(Other definitions for adversary that I've seen before include "Antagonist, opponent" , "Enemy or rival" , "Foe, opponent" , "Opponent or enemy" .)