Show hole under knickers, briefly (5)
I believe the answer is:
panto
'show' is the definition.
(pantomime)
'hole under knickers briefly' is the wordplay.
'hole' becomes 'O' (O looks like a hole).
'under' means one lot of letters go next to another (in a down clue, some letters go below others).
'knickers' becomes 'pants' (synonyms).
'briefly' means to remove the last letter.
'pants' with its final letter removed is 'pant'.
'o' put after 'pant' is 'PANTO'.
(Other definitions for panto that I've seen before include "In brief, traditional Christmas entertainment" , "festive entertainment" , "Slapstick theatre for children (abbr)" , "theatrical work" , "lowbrow entertainment" .)