Mares most spooked by monkeys (9)

I believe the answer is:

marmosets

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

'monkeys' is the definition.
(I know this)

'mares most spooked' is the wordplay.
'mares most' can be anagrammed to 'MARMOSETS'.
However, I am unsure how this is indicated.

'by' acts as a link.

Can you help me to learn more?

(Other definitions for marmosets that I've seen before include "Monkeys with silky coats and long tails" , "The monkeys could give Moms a rest" .)

I've seen this clue in the Sydney Morning Herald.

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