Chews up a thousand fishes (7)

I believe the answer is:

mangles

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'chews up' is the definition.
Although both the answer and definition are verbs in their -s form, I can't understand how they can define each other.

'a thousand fishes' is the wordplay.
'a thousand' becomes 'm' (Roman numeral for a thousand).
'fishes' becomes 'angles' (angling is a kind of fishing).
'm'+'angles'='MANGLES'

Can you help me to learn more?

(Another definition for mangles that I've seen is " Mutilates".)

I've seen this clue in The Globe and Mail.

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