Powerless when nebula spreads (6)
I believe the answer is:
unable
'powerless' is the definition.
(similar in meaning)
'nebula spreads' is the wordplay.
'spreads' indicates an anagram (I've seen 'spreading of' mean this).
'nebula' is an anagram of 'UNABLE'.
'when' acts as a 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)" .)