Source of free-flowing water, primarily less saline (8,4)
I believe the answer is:
artesian well
'source' is the definition.
(I have seen 'Source of water' mean 'artesian well' so perhaps 'source' could also mean 'artesian well')
'free-flowing water primarily less saline' is the wordplay.
'free flowing' indicates an anagram (letters flow around).
'primarily' says to take the initial letters.
The initial letter of 'less' is 'l'.
'water'+'l'+'saline'='waterlsaline'
'waterlsaline' is an anagram of 'ARTESIAN WELL'.
'of' is the link.
(Other definitions for artesian well that I've seen before include "device for getting water" , "supply of water" , "Bored source of water" , "Source of water with natural pressure" .)