Small horse that may be seen in a marshalling yard (7)

'a marshalling yard' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

'small horse that may be' is the wordplay.
I cannot really understand how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found in the answer.
'horse' could be 'hunter' (I've seen this before) and 'hunter' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

'seen in' acts as a link.

Can you help me to learn more?