Group supports students' uprising at dusk (6)
I believe the answer is:
sunset
'dusk' is the definition.
(period after sunset)
'group supports students' uprising' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'supports' is a charade indicator (letters next to each other) (bottom letters provide a support for the top ones).
'students'' becomes 'NUS' (I've seen this before).
'uprising' says the letters should be written backwards.
'nus' reversed gives 'sun'.
'set' after 'sun' is 'SUNSET'.
'at' acts as a link.
(Other definitions for sunset that I've seen before include "Time of day when the light begins to fail" , "When the light begins to fall below the horizon" , "End of the day, getting dark" , "After which day becomes night" , "Dusk" .)