Accepted a mate tying the knot presumably tested the water (4,3,6)
I believe the answer is:
took the plunge
'knot presumably tested the water' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'accepted a mate tying the' is the wordplay.
I cannot quite see how this works, but
'accepted' could be 'took' (I've seen this in another clue) and 'took' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for took the plunge that I've seen before include "Summoned up the courage" , "left the board, and > got married?" , "Made a risky, irreversible decision" .)