Admitted one may have had to pay it (8,3)
I believe the answer is:
entrance fee
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'admitted one may have had to pay' is the wordplay.
'admitted one may have' becomes 'entrance' (I can't justify this - if you can you should believe this answer much more).
'had' means one lot of letters go next to another.
'to pay' becomes 'fee'.
'entrance'+'fee'='ENTRANCE-FEE'
Can you help me to learn more?
(Other definitions for entrance fee that I've seen before include "Admission charge" , "Getting in charge" .)