Bono's trilby covers hole in head (7)
I believe the answer is:
nostril
'hole in head' is the definition.
(a hole in part of the head)
'bono's trilby covers' is the wordplay.
'covers' indicates a hidden word (I've seen 'covered' mean this (the answer is covered up by the clue)).
'NOSTRIL' is hidden in the letters of 'bonos trilby'.
(Other definitions for nostril that I've seen before include "Aperture in nose" , "Nose hole" , "Nasal passage" , "One of two holes in the head!" , "Breathing channel" .)