Being small, they need only three wheels (6)

I believe the answer is:

trikes

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'being small they need only' is the definition.
The definition suggests an adverb but the answer is not.

'three wheels' is the wordplay.
'three' becomes 'tri' (prefix meaning 'three').
'wheels' becomes 'kes' (I can't justify this - if you can you should believe this answer much more).
'tri'+'kes'='TRIKES'

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I've seen this clue in the Evening Standard.

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