Oddly, the Welsh river in England (4)
I believe the answer is:
tees
'river in england' is the definition.
(English river)
'oddly the welsh' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'the'+'welsh'='thewelsh'
The alternating letters of 'thewelsh' are 'TEES'.
(Other definitions for tees that I've seen before include "Sport items" , "River on which Middlesbrough stands" , "Starting places on the golf course" , "Flower" , "Golfer's supports; river" .)