Piece of spinach on a tortilla chip (5)
I believe the answer is:
nacho
'tortilla chip' is the definition.
(nacho is a kind of tortilla chip)
'piece of spinach on a' is the wordplay.
'piece of' indicates a hidden word.
'NACHO' is hidden within 'spinach on a'.
(Other definitions for nacho that I've seen before include "Mexican dish" , "Tortilla chip served with savoury toppings" , "Mexican snack item" , "Small piece of tortilla with a topping" , "Tortilla chip topped with cheese and spices" .)