In Ireland, a river used by southerners (4)

I believe the answer is:

erne

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'in ireland a river' is the definition.
I can't judge whether this definition defines the answer.

'used by southerners' is the wordplay.
'used by' indicates a hidden word.
'ERNE' can be found hidden inside 'southerners'.

Can you help me to learn more?

(Other definitions for erne that I've seen before include "Sea bird: lough in Ulster" , "Great lake in Fermanagh with lots of islands" , "European sea eagle" , "Sea-eagle in an Irish lake" , "Marine flyer" .)

I've seen this clue in the Evening Standard.

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