The well off won't be on it (4,5)
I believe the answer is:
sick leave
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sick leave that I've seen before include "Time for workers to become better" , "Ill time off work" , "off break?" , "Time off when 11 ac" , "when not working on this" .)