Second class journey for groom's partner (5)
I believe the answer is:
bride
'groom's partner' is the definition.
(I've seen this before)
'second class journey' is the wordplay.
'second class' becomes 'b'.
'journey' becomes 'ride' (riding is a kind of journeying).
'b'+'ride'='BRIDE'
'for' is the link.
(Other definitions for bride that I've seen before include "She's just about to be married" , "The Woman In White, perhaps?" , "Marriageable woman" , "one going to church or office?" , "new union member" .)