Marriage in trade group (5)
I believe the answer is:
union
'marriage' is the definition.
(I know that marriage can be written as union)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for union that I've seen before include "Coalescence, join as one" , "workers' co-operative" , "Big old pearl" , "The joining of two or more together" , "Climbing plant" .)