Nutrient in spinach (4)
I believe the answer is:
iron
'nutrient in spinach' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for iron that I've seen before include "Strong metal; golf club" , "great strength" , "The one who was 'not for turning'" , "Main element in steel" , "Box-shaped instrument of torture -- no meridian (anag)" .)