Popeye's energy source (7)
I believe the answer is:
spinach
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">popeye's energy source</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for spinach that I've seen before include "Vegetable leaves" , "Leafy vegetable" , "Some greens" , "Blackboard stand" , "Dark green vegetable" .)