Piece of fish, I note, not unlimited (6)
I believe the answer is:
finite
'unlimited' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?
'piece of fish i note not' is the wordplay.
'piece of' says to take the initial letters.
'note not' becomes 'nite' (I am not sure about this - if you are sure you should believe this answer much more).
The initial letter of 'fish' is 'f'.
'f'+'i'+'nite'='FINITE'
Can you help me to learn more?
(Other definitions for finite that I've seen before include "Having set limits" , "Bounded or limited" , "Having limit, not endless" , "Circumscribed" , "Bounded, having limits" .)