Sign fluctuates, but not initially (5)
I believe the answer is:
aries
'sign' is the definition.
(star sign)
'fluctuates but not initially' is the wordplay.
'fluctuates' becomes 'varies' (I've seen this in another clue).
'but not initially' means to remove the first letter.
'varies' with its initial letter taken away is 'ARIES'.
(Other definitions for aries that I've seen before include "Sheep" , "Ram star sign" , "First sign of the zodiac" , "Constellation with sign of the ram" , "Zodiac ram" .)