Increase needed to cover fish supplement (9)
I believe the answer is:
surcharge
'supplement' is the definition.
(I've seen this before)
'increase needed to cover fish' is the wordplay.
I cannot quite see how this works, but
'fish' could be 'char' (char is a kind of fish) and 'char' is found within the answer.
'increase' could be 'surge' (surge is a kind of increase) and 'surge' is located in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for surcharge that I've seen before include "Excess fee" , "Additional cost" , "More to pay" , "supplement" , "extra amount" .)