Marriage metaphor (4)
I believe the answer is:
knot
'marriage metaphor' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for knot that I've seen before include "1 (nautical) mph" , "Measure of speed at sea" , "A bowline or a reef" , "Small group (of people); tied rope" , "Ligature" .)