Crossword Genius

Transport workers whose motion is carried by four members (6,2,6)

Ross

I believe the answer is:

beasts of burden

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This might be a double definition.

'transport workers' is the first definition.
I know that 'transport' relates to this answer.

'motion is carried by four members' is the second definition.
I know that 'motion' relates to this answer.

'whose' acts as a link.

Can you help me to learn more?

(Other definitions for beasts of burden that I've seen before include "Eg, donkeys, camels, etc" , "Mules, maybe" .)

I've seen this clue in The Telegraph.
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