Midweek, briefly takes the plunge (4)
I believe the answer is:
weds
This clue is a double definition.
'midweek briefly' is the first definition.
(short for Wednesday)
'takes the plunge' is the second definition.
(to take the plunge can mean to get married)
(Other definitions for weds that I've seen before include "Joins together" , "Gets married" , "day?" , "Marries" , "Makes match" .)