Mostly, she may be in a hole! (5)
I believe the answer is:
peggy
'be in a hole' is the definition.
The definition suggests a singular noun which matches the answer.
'mostly she' is the wordplay.
I cannot really understand how this works, but
'she' could be 'peg' and 'peg' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'may' is the link.
Can you help me to learn more?
(Other definitions for peggy that I've seen before include "- - Lee, popular singer" , "Dolly < bird" , "one associated with Sue" .)