Christmas entertainment (5)
(Other definitions for panto that I've seen before include "cross-dressing performance" , "In brief, traditional Christmas entertainment" , "Christmas show" , "Christmas entertainment (abbr.)" , "lowbrow entertainment" .)
'christmas entertainment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pantomime that I've seen before include "Traditional Christmas stage show with Dame" , "Theatrical entertainment (usually at Christmas)" , "Mime on tap in children's show" , "eg 'Mother Goose'" , "Farcical situation" .)