Crossword Genius

Arrive on locomotive's caboose? (4)

Ross

I believe the answer is:

lobe

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'caboose?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

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

Can you help me to learn more?

(Other definitions for lobe that I've seen before include "Roundish projecting part" , "Rounded projection of a larger structure" , "Base of ear" , "flap" , "Lower part of the ear" .)

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