Chip in spinach omelette (5)
I believe the answer is:
nacho
'chip' is the definition.
(I've seen this before)
'in spinach omelette' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NACHO' is hidden in the letters of 'spinach omelette'.
(Other definitions for nacho that I've seen before include "Tortilla chip served with savoury toppings" , "Small piece of tortilla with a topping" , "Mexican bite" , "Mexican corn chip eaten with cheese, etc" , "Mexican snack item" .)