His occupation is subject to contract (11)
I believe the answer is:
leaseholder
'his occupation' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'subject to contract' is the wordplay.
'subject' becomes 'holder' (I am not sure about this - if you are sure you should believe this answer much more).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'contract' becomes 'lease' (lease is a kind of contract).
'holder' put after 'lease' is 'LEASEHOLDER'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for leaseholder that I've seen before include "Person with property rights" , "One renting property" , "Tenant" .)