Neil changed course (4)
I believe the answer is:
line
'course' is the definition.
('line' can be a synonym of 'course')
'neil changed' is the wordplay.
'changed' indicates an anagram.
'neil' with letters rearranged gives 'LINE'.
(Other definitions for line that I've seen before include "Wrinkle; row" , "Dress" , "Winter sports slope" , "Phone connection; queue" , "short note" .)