Custodian of a grounded jumbo (6)

I believe the answer is:

mahout

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'custodian' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mahout that I've seen before include "Elephant keeper/driver" , "Keeper and driver of elephants" , "He looks after elephants in India" , "Jumbo rider" , "jumbo? I'll see to it" .)

I've seen this clue in The Globe and Mail.

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