60s rebel nutcase may have gone off this (6)
I believe the answer is:
rocker
This clue is a double definition.
'60s rebel' is the first definition.
(member of the rocker subculture)
'nutcase may have gone off this' is the second definition.
(as in the phrase 'off one's rocker')
(Other definitions for rocker that I've seen before include "Mod's 1960s rival - curved support under chair legs" , "Opponent of mod" , "special chair" , "Someone who likes form of pop" , "Moving chair" .)