"Batter” in fish — don't buy it (5,1,3)
I believe the answer is:
smell a rat
'don't buy it' is the definition.
(I've seen this in another clue)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for smell a rat that I've seen before include "Have suspicion something is afoot" , "Suspect something wrong" , "Suspect deception" , "Detect something suspicious" , "Find something fishy" .)