The under part of W. Glamorgan? (5)

I believe the answer is:

neath

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

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

(Other definitions for neath that I've seen before include "Town NE of Swansea" , "Welsh team" , "Lower than - a town in West Glamorgan" , "Under (poetic)" , "Under, poetically" .)

I've seen this clue in the Evening Standard.

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