Panto character’s mistake, briefly entering orchestra area (7)
I believe the answer is:
pierrot
'panto character's' is the definition.
(stock pantomime character)
'mistake briefly entering orchestra area' is the wordplay.
'mistake' becomes 'error' (synonyms).
'briefly' means to remove the last letter.
'entering' is an insertion indicator.
'orchestra area' becomes 'pit' (I've seen this in another clue).
'error' with its final letter taken away is 'erro'.
'erro' placed inside 'pit' is 'PIERROT'.
(Other definitions for pierrot that I've seen before include "Clown from French pantomime with white face and costume - tripe or what?" , "In French panto, clown with white face and baggy costume" , "Male clown from French pantomime" , "Seaside clown" , "I report on French pantomime clown" .)