Reprimand actors at one entrance (9)
I believe the answer is:
castigate
'reprimand' is the definition.
(castigating is a kind of reprimanding)
'actors at one entrance' is the wordplay.
'actors' becomes 'cast' (group of actors).
'at' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'entrance' becomes 'gate' (type of entrance).
'cast'+'i'+'gate'='CASTIGATE'
(Other definitions for castigate that I've seen before include "Take to task" , "Censure severely" , "Chastise - tacit sage (anag)" , "Blast" , "Berate" .)