Fish reduced by Scrooge (10)
I believe the answer is:
cheapskate
'scrooge' is the definition.
(cheapskate is a kind of scrooge)
'fish reduced' is the wordplay.
I cannot quite understand how this works, but
'fish' could be 'skate' (skate is a kind of fish) and 'skate' is present in the answer.
The remaining letters 'cheap' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'by' is the link.
Can you help me to learn more?
(Other definitions for cheapskate that I've seen before include "Miserly person" , "he won't pay the going rate" , "Tightwad" , "One's tight" , "Mean person" .)