Powerless when nebula spreads (6)
I believe the answer is:
Here is my best explanation:
'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 "Not in a position (to)", "Impotent", "Lacking the required capacity", "Powerless", "Not fit or competent", "Not capable", "Incapable", "Lacking the skill", "Not having the neccessary means" and "Not strong enough".)