Anger when changing gear (4)
I believe the answer is:
rage
'anger' is the definition.
(rage is a kind of anger)
'changing gear' is the wordplay.
'changing' indicates an anagram.
'gear' with letters rearranged gives 'RAGE'.
'when' is the link.
(Other definitions for rage that I've seen before include "Cult - passion" , "Uncontrolled anger" , "Mania" , "Violent anger, maybe on road" , "Wrath" .)