He admits he doesn't have to pay the rent (5)
I believe the answer is:
owner
'he admits he doesn't' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'have to pay the rent' is the wordplay.
'have' becomes 'own' (to have something is to own it).
'to pay the rent' becomes 'er' (I am not sure about this - if you are sure you should believe this answer much more).
'own'+'er'='OWNER'
Can you help me to learn more?
(Other definitions for owner that I've seen before include "landlord" , "possessive type" , "Keeper" , "Holder, possessor" , "Title holder?" .)