First of six lives for sibling (3)
I believe the answer is:
sis
'for sibling' is the definition.
(short for sister)
'first of six lives' is the wordplay.
'first of six' becomes 's' (1st letter of 'six').
'lives' becomes 'is' ('be' can be a synonym of 'live').
's'+'is'='SIS'
(Other definitions for sis that I've seen before include "Sibling (informal)" , "Secret Intelligence Service (abbreviation)" , "Family member (informal)" , "Girl" , "spies ..." .)