Defeat regularly upset supporter? (3)
I believe the answer is:
tee
'supporter?' is the definition.
(a golf tee acts as a support)
'defeat regularly upset' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'upset' says the letters should be written backwards.
The alternating letters of 'defeat' are 'eet'.
'eet' back-to-front is 'TEE'.
(Other definitions for tee that I've seen before include "support of leisure activity" , "Drive off it, half-tittering" , "Golfing support" , "Start of a hole in golf" , "Part of course" .)