What the swimmers might do to join The Union (4,3,6)
I believe the answer is:
take the plunge
'swimmers might do to join the union' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'what the' is the wordplay.
I cannot really understand how this works, but
an anagram of 'the' is 'eth' which is within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for take the plunge that I've seen before include "Bungee jumper has to" , "to leave the board" , "Eventually commit oneself" , "Get married (informal)" , "do a bungee jump?" .)