Transient, our agenda involves train (9)
I believe the answer is:
entourage
'train' is the definition.
(a train of people)
'transient our agenda involves' is the wordplay.
'involves' indicates a hidden word.
'ENTOURAGE' is hidden within 'transient our agenda'.
(Other definitions for entourage that I've seen before include "Support team" , "Supportive group" , "Staff" , "company" , "Group of attendants" .)