Butter turns up in a staple diet (3)
I believe the answer is:
pat
'butter' is the definition.
(a pat can mean a lump of butter)
'turns up in a staple diet' is the wordplay.
'turns up' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'tap' is hidden within 'staple diet'.
'tap' reversed gives 'PAT'.
(Other definitions for pat that I've seen before include "'Light touch, on the back say (3)'" , "Light touch or stroke" , "Portion of butter" , "Affectionate gesture - postman" , "Apt for tender touch" .)