Shine unsteadily (7)
I believe the answer is:
flicker
'shine unsteadily' is the definition.
(synonyms)
This is all the clue.
(Other definitions for flicker that I've seen before include "Vary rapidly in brightness" , "Shine unsteadily; waver" , "Burn or shine unsteadily" , "Woodpecker" , "Glimmer" .)