Willing extra fish in frozen condition to be docked (7)
I believe the answer is:
codicil
'willing extra' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'fish in frozen condition to be docked' is the wordplay.
'fish' becomes 'cod' (type of fish).
'in' means one lot of letters goes inside another.
'frozen' becomes 'icl' (I can't justify this - if you can you should give a lot more credence to this answer).
'condition' becomes 'if' (as in 'a big if').
'to be docked' means to remove the last letter (to dock can mean to remove the tail of an animal).
'if' with its last letter removed is 'i'.
'icl' enclosing 'i' is 'icil'.
'cod'+'icil'='CODICIL'
Can you help me to learn more?
(Other definitions for codicil that I've seen before include "Appendix to a will" , "Modifier to will" , "Addendum to will" , "Will's rider" , "part of will" .)