Grazing for a group of horses? (5)

I believe the answer is:

field

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

'group of horses?' is the definition.
'field' can be an answer for 'group' (field is a kind of group). I am unsure of the 'of horses?' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for field that I've seen before include "Piece of farmland" , "Games area" , "Area of knowledge, for game" , "Area for crops" , "Sphere of endeavour" .)

I've seen this clue in the Evening Standard.

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