Reason for which the river at Chepstow is mentioned (3)
I believe the answer is:
why
'reason for which' is the definition.
(why something is the case)
'river at chepstow is mentioned' is the wordplay.
'river at chepstow' becomes 'Wye' (River Wye in Wales).
'is mentioned' shows a homophone (sound like).
'wye' sounds like 'WHY'.
'the' is the link.
(Other definitions for why that I've seen before include "Question" , "How come?" , "For what reason?" .)