Witch in a Sacha Guitry classic (3)
I believe the answer is:
'witch' is the definition.
(I know that witch can be written as hag)
'in a sacha guitry classic' is the wordplay.
'in' says the answer is hidden in the clue.
'HAG' can be found hidden inside 'sacha guitry classic'.
(Other definitions for hag that I've seen before include "Crone" , "Witch or ugly old woman" , "Old witch-like woman" .)