Heard cook grating (6)
I believe the answer is:
grille
'grating' is the definition.
(grille is a kind of grating)
'heard cook' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'cook' becomes 'grill' (grilling is a form of cooking).
'grill' sounds like 'GRILLE'.
(Other definitions for grille that I've seen before include "It affords ventilation" , "Frame of bars" , "Part of an air-cooling system" , "Protective metal framework" , "Grating, metal screen" .)