One in union to travel on bike at first (5)
I believe the answer is:
bride
'one in union' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'travel on bike at first' is the wordplay.
'travel' becomes 'ride' (eg to ride a bus).
'on' is a charade indicator (letters next to each other).
'at first' suggests taking the first letters.
The initial letter of 'bike' is 'b'.
'ride' after 'b' is 'BRIDE'.
'to' is the link.
Can you help me to learn more?
(Other definitions for bride that I've seen before include "She's just about to be married" , "The Woman In White, perhaps?" , "Be rid of the married lady" , "Woman getting wed" , "Newlywed" .)