One of three born together (7)
I believe the answer is:
triplet
'one of three born together' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for triplet that I've seen before include "Set of three things" , "Handful of notes" , "One of three born at the same time" , "One of several arriving together" , "One of a set of three" .)