Ruby cut fish (3,6)
I believe the answer is:
red mullet
'ruby cut fish' is the definition.
'red mullet' can be an answer for 'fish' (red mullet is a kind of fish). I am not certain of the 'ruby cut' bit.
'ruby cut fish' is the wordplay.
'ruby' becomes 'red' (synonyms).
'cut fish' becomes 'mullet' (mullet is a kind of fish. I am not sure about the 'cut' bit.).
'red'+'mullet'='RED MULLET'
Can you help me to learn more?
(Other definitions for red mullet that I've seen before include "Tasty swimmer" , "There's something fishy" , "Brightly coloured food fish" , "one's gathered in net" , "Fish -- dull meter (anag)" .)