One of three allowed on journey (7)
I believe the answer is:
triplet
'one of three' is the definition.
(one of a set of triplets)
'allowed on journey' is the wordplay.
'allowed' becomes 'let' (to let is to allow or permit).
'on' means one lot of letters go next to another.
'journey' becomes 'trip' (trip is a kind of journey).
'let' put after 'trip' is 'TRIPLET'.
(Other definitions for triplet that I've seen before include "Member of large family" , "One of a set of three" , "Baby with two similar siblings" , "One of three children" , "Group of three equal notes" .)