Crossword Genius

Run racehorses together (6)

Ross

I believe the answer is:

string

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

'run racehorses' is the definition.
'string' can be an answer for 'racehorses' (I've seen this before). I'm unsure of the 'run' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for string that I've seen before include "Hard to say how long a piece of it is" , "tier" , "Part of racket" , "How long could a piece of it be?" , "Group of racers" .)

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