Crossword Genius

What destructive mice get up to with the fish (8)

Ross

I believe the answer is:

mischief

I'm a little stuck... Click here to teach me more about this clue!

'the fish' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe there's an association between them I don't understand?

'what destructive mice get up to' is the wordplay.
I cannot quite understand how this works, but
'up' could be 'is' (I've seen this in other clues) and 'is' is located in the answer.
This explanation may well be incorrect...

'with' acts as a link.

Can you help me to learn more?

(Other definitions for mischief that I've seen before include "Black . . . . . . . . (Evelyn Waugh)" , "Devilment, impishness" , "injury" , "Naughtiness, devilment" , "Naughtiness, trouble" .)

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