Source of water in Wales later abandoned (8,4)
I believe the answer is:
artesian well
'source of water' is the definition.
(I've seen this before)
'in wales later abandoned' is the wordplay.
'abandoned' indicates an anagram (the original order is abandoned).
'in'+'wales'+'later'='inwaleslater'
'inwaleslater' anagrammed gives 'ARTESIAN WELL'.
(Other definitions for artesian well that I've seen before include "supply of water" , "Bored source of water" , "Source of water with natural pressure" , "device for getting water" .)