Ring under nut (6)
I believe the answer is:
washer
'ring under nut' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for washer that I've seen before include "Plumber often wants it" , "Ring placed between two surfaces to prevent leakage, as in tap" , "Leak-stopping ring" , "Baptist" , "Rendered sightless" .)