A fishy diversion (3,7)
I believe the answer is:
red herring
'diversion' is the definition.
(I know that red herring is a type of diversion)
'a fishy' is the wordplay.
I cannot really see how this works, but
'fishy' could be 'herring' (I've seen this before) and 'herring' is found in the answer.
The remaining letters 'red' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for red herring that I've seen before include "False lead" , "What distracts" , "It may well put you off" , "fish" , "Diversion intended to distract attention from the main issue" .)