One with instant siblings (7)
I believe the answer is:
triplet
'one with instant siblings' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for triplet that I've seen before include "One of several arriving together" , "One of three at the same birth" , "Handful of notes" , "Member of large family" , "Set of three things" .)