Inland fish found in a triangle (8,5)

I believe the answer is:

interior angle

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'a triangle' is the definition.
The definition and answer can be both shapes as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

'inland fish' is the wordplay.
'inland' becomes 'interior' (similar in meaning).
'fish' becomes 'angle' (angle can mean to fish).
'interior'+'angle'='INTERIOR ANGLE'

'found in' is the link.

Can you help me to learn more?

(Another definition for interior angle that I've seen is " Euclid was familiar with?".)

I've seen this clue in The Daily Express.

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