Which horses are featured in a manuscript? (5)

I believe the answer is:

mares

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

'which horses' is the definition.
I can't judge whether this definition defines the answer.

'are featured in a manuscript?' is the wordplay.
'featured in' means one lot of letters goes inside another.
'a manuscript?' becomes 'ms' (abbreviation for manuscript).
'are' placed into 'ms' is 'MARES'.

Can you help me to learn more?

(Another definition for mares that I've seen is " Female horses".)

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

Want a hint initially instead of a full solution? Install my app