Express disapproval with ruler's reservation (7)
I believe the answer is:
booking
'reservation' is the definition.
(synonyms)
'express disapproval with ruler's' is the wordplay.
'express disapproval' becomes 'boo' (as in a crowd booing).
'with' means one lot of letters go next to another.
'ruler' becomes 'king' (king is a kind of ruler).
'boo'+'king'='BOOKING'
(Other definitions for booking that I've seen before include "Punishment for offending player" , "Appointment" , "Reservation" .)