Comfortless role in school sickbay (7)
I believe the answer is:
spartan
'comfortless' is the definition.
(spartan can mean lacking in comfort)
'role in school sickbay' is the wordplay.
'role' becomes 'part' (eg a part or role in a film).
'in' is an insertion indicator.
'school sickbay' becomes 'san' (short for sanatorium, as in a boarding school).
'part' placed inside 'san' is 'SPARTAN'.
(Other definitions for spartan that I've seen before include "Frugal" , "Indifferent to comfort" , "Of conditions, simple and harsh as for ancient Greeks" , "Ascetic - ancient Greek" , "Plain" .)