Collection of beach pebbles, not hard — one only (6)

I believe the answer is:

single

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'one only' is the definition.
(I've seen this before)

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

(Other definitions for single that I've seen before include "Sort of person" , "Solo" , "A hit for one run in cricket" , "Still miss" , "Like bachelor" .)

I've seen this clue in The Times.

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