Happen to catch free union member (5)
I believe the answer is:
bride
'union member' is the definition.
(I've seen this in other clues)
'happen to catch free' is the wordplay.
'happen' becomes 'be' (I've seen this in another clue).
'to catch' indicates putting letters inside.
'free' becomes 'rid' (to be free of something is to be rid of it).
'be' enclosing 'rid' is 'BRIDE'.
(Other definitions for bride that I've seen before include "one going to church or office?" , "Be rid of the married lady" , "woman in front of train" , "One getting married" , "She's just about to be married" .)