Spring changes course (6)
I believe the answer is:
source
'spring' is the definition.
(I've seen this before)
'changes course' is the wordplay.
'changes' indicates an anagram.
'course' is an anagram of 'SOURCE'.
(Other definitions for source that I've seen before include "Place or person where something begins" , "Origin or starting point" , "Origin; head of stream" , "Well" , "Origin as of a river" .)