Mark is male in very deep sleep (5)
I believe the answer is:
comma
'mark' is the definition.
(comma is a kind of mark)
'male in very deep sleep' is the wordplay.
'male' becomes 'm' (common abbreviation).
'in' means one lot of letters goes inside another.
'very deep sleep' becomes 'coma'.
'm' going into 'coma' is 'COMMA'.
'is' acts as a link.
(Other definitions for comma that I've seen before include "Kind of butterfly" , "Butterfly with orange and brown wings" , "Common punctuation park" , "repeatedly found in lists" , "its use may be divisive" .)