Groomsmen regularly rebelled (4)
I believe the answer is:
rose
'rebelled' is the definition.
(I've seen this in other clues)
'groomsmen regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'groomsmen' are 'ROSE'.
(Other definitions for rose that I've seen before include "came up" , "Revolted" , "Moved upwards for the flower" , "Flower; went upwards" , "13 flower" .)