Sign changes, but not initially (5)
I believe the answer is:
aries
'sign' is the definition.
(star sign)
'changes but not initially' is the wordplay.
'changes' becomes 'varies' ('vary' can be a synonym of 'change').
'but not initially' means to remove the first letter.
'varies' with its initial letter taken off is 'ARIES'.
(Other definitions for aries that I've seen before include "A sign from Heaven" , "star group" , "House" , "Ram star sign" , "Stars" .)