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