Key-ring with a key on it in the shape of a fish (3)
I believe the answer is:
cod
'shape of a fish' is the definition.
'cod' can be an answer for 'fish' (type of fish). I am not certain of the 'shape of a' bit.
'key-ring with a key on it' is the wordplay.
'key' becomes 'c' (musical key).
'ring' becomes 'O' (letter resembling a ring).
'with' means one lot of letters go next to another.
'a key' becomes 'd' (musical key).
'on it' is a charade indicator (letters next to each other).
'c'+'o'+'d'='COD'
'in the' is the link.
Can you help me to learn more?
(Other definitions for cod that I've seen before include "swimmer" , "faked" , "Not a real" , "catch!" , "One caught in the deep maybe" .)