To hide one disc in another is fishy! (3)
I believe the answer is:
cod
'fishy' is the definition.
(type of fish)
'to hide one disc in another' is the wordplay.
'to hide' is an insertion indicator.
'one disc' becomes 'O' (disc-shaped letter).
'in another' becomes 'cd' (I am not sure about this - if you are sure you should believe this answer much more).
'o' going inside 'cd' is 'COD'.
'is' is the link.
Can you help me to learn more?
(Other definitions for cod that I've seen before include "Play joke on" , "Payment arrangement" , "Tasty fish or slangy nonsense" , "swimmer" , "Fish; fake (colloq.)" .)