River boundary between the US and Mexico (3,6)

I believe the answer is:

rio grande

'river boundary between the us and mexico' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for rio grande that I've seen before include "Long river" , "running water" , "River boundary between the US and Mexico" , "US-Mexico border river" , "Mexico-USA border" .)

I've seen this clue in The Irish Times.

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