Interrogated endlessly: that's grating (6)
I believe the answer is:
grille
'grating' is the definition.
(a grille is a grating made of metal bars)
'interrogated endlessly' is the wordplay.
'interrogated' becomes 'grilled' (I've seen this before).
'endlessly' means to remove the last letter.
'grilled' with its last letter taken away is 'GRILLE'.
'that's' acts as a link.
(Other definitions for grille that I've seen before include "Grating, metal screen" , "Protective lattice" , "Partition" , "Frame of bars" , "Part of an air-cooling system" .)