Rejected help at supermarket bagging essential food item (6)
I believe the answer is:
staple
'essential food item' is the definition.
'rejected help at supermarket bagging' is the wordplay.
'rejected' says the letters should be written backwards.
'bagging' indicates a hidden word.
'elpats' is hidden within 'help at supermarket'.
'elpats' backwards is 'STAPLE'.
(Other definitions for staple that I've seen before include "One securing hasp; principal" , "Paper securer; main" , "Clip" , "means of keeping papers together" , "A wire paper fastener for plates" .)