Fish discovered, cryptically has gone off (7)
I believe the answer is:
gudgeon
'fish' is the definition.
(gudgeon is a kind of fish)
'cryptically has gone off' is the wordplay.
'cryptically' becomes 'gud' (I can't justify this - if you can you should give a lot more credence to this answer).
'has' is a charade indicator (letters next to each other).
'off' indicates an anagram.
'gone' is an anagram of 'geon'.
'gud'+'geon'='GUDGEON'
'discovered' acts as a link.
Can you help me to learn more?
(Other definitions for gudgeon that I've seen before include "Swimmer" , "Small freshwater fish" , "Fish, or person easily cheated" , "Small carp-like fish" , "angler's bait" .)