Popeye's energy source (7)
I believe the answer is:
spinach
'popeye's energy source' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for spinach that I've seen before include "Vegetable leaves" , "Leafy vegetable" , "Some greens" , "Blackboard stand" , "Dark green vegetable" .)