One of three born at the same time (7)
I believe the answer is:
triplet
'one of three born at the same time' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for triplet that I've seen before include "Not the only baby in the group" , "One of three at the same birth" , "One of a small group" , "Baby with two similar siblings" , "One of a set of three" .)