I must change after hours to cause a stir (9)
I believe the answer is:
stimulate
'a stir' is the definition.
(I know that stir can be written as stimulate)
'i must change after hours' is the wordplay.
'change' indicates anagramming the letters.
'after hours' becomes 'late' (I've seen this in another clue).
'i'+'must'='imust'
'imust' is an anagram of 'stimu'.
'stimu'+'late'='STIMULATE'
'to cause' acts as a link.
(Other definitions for stimulate that I've seen before include "Excite the feelings of" , "Encourage" , "Animate, excite" , "Kindle?" , "Incite to action" .)