Powerless when nebula spreads (6)
I believe the answer is:
unable
'powerless' is the definition.
('unable' can be similar in meaning to 'powerless')
'nebula spreads' is the wordplay.
'spreads' is an anagram indicator (I've seen 'spread' mean this).
'nebula' with letters rearranged gives 'UNABLE'.
'when' is the link.
(Other definitions for unable that I've seen before include "Incompetent or powerless" , "Not capable" , "Lacking the required capacity" , "Lacking the necessary skills" , "Lacking power (to do something)" .)