An extra fish, I see, for one pound (7)
I believe the answer is:
codicil
'an extra' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'fish i see for one pound' is the wordplay.
'fish' becomes 'cod' (type of fish).
'see' becomes 'C' (the word for the letter, according to Chambers).
'for' means one lot of letters go next to another.
'one pound' becomes 'Il'.
'cod'+'i'+'c'+'il'='CODICIL'
Can you help me to learn more?
(Other definitions for codicil that I've seen before include "Additional item appended to a will" , "Addendum to will" , "Appendix to a will" , "part of will" , "Portion added on to a will" .)