Journey permitted for one of three (7)
I believe the answer is:
triplet
'one of three' is the definition.
(one of a set of triplets)
'journey permitted' is the wordplay.
'journey' becomes 'trip' (trip is a kind of journey).
'permitted' becomes 'let' (to let something happen is to permit it).
'trip'+'let'='TRIPLET'
'for' acts as a link.
(Other definitions for triplet that I've seen before include "One of three born together" , "sibling?" , "One of a group of three" , "Handful of notes" , "One of three at the same birth" .)