Detail about a pair of skiffs in a river (7)

I believe the answer is:

specify

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'detail' is the definition.
(I know that detail is a more specific form of specify)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for specify that I've seen before include "State definitely" , "Nominate" , "Mention particularly" , "Identify clearly and definitely" , "Set down as required" .)

I've seen this clue in The Daily Express.

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