Transport used by a wise man - or a fool (3)

I believe the answer is:

ass

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'a fool' is the definition.
(both can mean a stupid person)

I cannot understand how the remainder of the clue works.

(Other definitions for ass that I've seen before include "Hardy and sure-footed animal" , "slow-witted person" , "Horse-type animal" , "One easily had" , "'The law is a - - - a idiot' (Mr Bumble)" .)

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

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