A smart-alecky fish? That ain't right (5,5)
I believe the answer is:
acute angle
'ain't right' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'a smart-alecky fish?' is the wordplay.
'smart-alecky' becomes 'cute' (I've seen this in another clue).
'fish?' becomes 'angle' (angle can mean to fish).
'a'+'cute'+'angle'='ACUTE ANGLE'
'that' is the link.
Can you help me to learn more?
(Other definitions for acute angle that I've seen before include "tighter corner" , "That can't be right - it's smaller" , "That's not right" , "Meeting point" .)