Withdraw pamphlet supporting monarch's return (7)
I believe the answer is:
retract
'withdraw' is the definition.
(thesaurus)
'pamphlet supporting monarch's return' is the wordplay.
'pamphlet' becomes 'tract' ('tract' can be a synonym of 'pamphlet').
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'return' says the letters should be written in reverse.
'er' back-to-front is 're'.
'tract' after 're' is 'RETRACT'.
(Other definitions for retract that I've seen before include "Withdraw one's statement" , "Pull back (statements or claws, perhaps)" , "Reverse" , "Abjure" , "Eat one's words" .)