About to unite, possibly forming a train (7)
I believe the answer is:
retinue
'a train' is the definition.
(people following someone in a procession)
'about to unite possibly' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to' is a charade indicator (letters next to each other).
'possibly' is an anagram indicator.
'unite' with letters rearranged gives 'tinue'.
're'+'tinue'='RETINUE'
'forming' acts as a link.
(Other definitions for retinue that I've seen before include "Group of attendants for VIP" , "company" , "Attendants, followers" , "Suite - train" , "Group following an important person" .)