Spanner's good held by new union member (6)
I believe the answer is:
bridge
'spanner's' is the definition.
(I've seen this before)
'good held by new union member' is the wordplay.
'good' becomes 'g' (abbreviation).
'held by' is an insertion indicator.
'new union member' becomes 'bride' (I've seen this before).
'g' inserted inside 'bride' is 'BRIDGE'.
(Other definitions for bridge that I've seen before include "Viaduct" , "Spanning structure to allow passage" , "going across river?" , "Span or connect" , "Captain's area on a ship" .)