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' means one lot of letters goes inside another.
'old queen's' becomes 'ER's' (Elizabeth Regina, ie Queen Elizabeth).
'vary' going around '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" .)