Blunder allowed in one of three (7)
I believe the answer is:
triplet
'one of three' is the definition.
(I've seen this before)
'blunder allowed' is the wordplay.
'blunder' becomes 'trip' (trip is a kind of blunder).
'allowed' becomes 'let' (I've seen this before).
'trip'+'let'='TRIPLET'
'in' is the link.
(Other definitions for triplet that I've seen before include "sibling?" , "Not the only baby in the group" , "One of three children" , "One of a group of three" , "Baby with two similar siblings" .)