Grating last of cheese to go under cooker (6)
I believe the answer is:
grille
'grating' is the definition.
(a grille is a grating made of metal bars)
'last of cheese to go under cooker' is the wordplay.
'last of' says to take the final letters.
'to go under' says to put letters next to each other (in a down clue, some letters go below others).
'cooker' becomes 'grill' (device for grilling food).
The last letter of 'cheese' is 'e'.
'e' put after 'grill' is 'GRILLE'.
(Other definitions for grille that I've seen before include "A metal grating in door or window" , "It affords ventilation" , "Metal lattice-work" , "Frame of bars" , "Metal lattice" .)