Mark is male in very deep sleep (5)
I believe the answer is:
Here is my best explanation:
'mark' is the definition.
(comma is a kind of mark)
'male in very deep sleep' is the subsidiary indication.
'male' becomes 'm' (common abbreviation).
'in' indicates putting letters inside.
'very deep sleep' becomes 'coma'.
'm' placed inside 'coma' is 'comma'.
'is' is the link.
(Other definitions for comma that I've seen before include "Mark for smallest division of a sentence", "identifying place to breathe", "repeatedly found in lists", "Brief stop", ", which gives us pause", "Butterfly - punctuation mark", "Butterfly with orange and brown wings", "I signal a break", "its use may be divisive" and "breathtaking marking?".)