Sign that is in half of sitars (5)
I believe the answer is:
aries
'sign' is the definition.
(star sign)
'that is in half of sitars' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'in' indicates putting letters inside.
'half of' suggests halving the letters.
'sitars' with half the letters taken is 'ars'.
'ie' inserted inside 'ars' is 'ARIES'.
(Other definitions for aries that I've seen before include "star group" , "First sign of the zodiac" , "Zodiacal sign also known as the Ram" , "Stars" , "Constellation with sign of the ram" .)