This may be required to enter an empty ground (7)
I believe the answer is:
payment
'this may be required to enter' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'an empty ground' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'an'+'empty'='anempty'
'anempty' anagrammed gives 'PAYMENT'.
Can you help me to learn more?
(Other definitions for payment that I've seen before include "Discharge of debt" , "Discharge of an obligation with money" , "Settlement, remittance" , "Reward for work" , "Money (for goods)" .)