Should be anything one hears (5)
I believe the answer is:
ought
'should be' is the definition.
(I've seen this before)
'anything one hears' is the wordplay.
'anything' becomes 'aught' (I've seen this in another clue).
'one hears' shows a homophone (sound like) (I've seen 'heard of' mean this).
'aught' is a homophone of 'OUGHT'.
(Other definitions for ought that I've seen before include "Be logically necessary" , "Tough (anag)" , "Anything at all (archaic)" , "Be bound" , "Should, had, better" .)