If a man is willing this may be added (7)
I believe the answer is:
codicil
'willing this may be added' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'if a man' is the wordplay.
I cannot quite understand how this works, but
'man' could be 'codi' and 'codi' is located in the answer.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for codicil that I've seen before include "Portion added on to a will" , "part of will" , "Additional item appended to a will" , "Addition to a will" , "Will's supplement" .)