Crossword Genius

In getting a horse out, he's of service (6)

Ross

I believe the answer is:

marine

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But I can't explain it at all! Could you help me understand?

(Other definitions for marine that I've seen before include "Soldier associated with a navy" , "Soldier-sailor" , "Relating to shipping or naval matters" , "Soldier serving on land and sea" , "jolly" .)

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