Sound unstressed after chief visiting was abused (5)
I believe the answer is:
schwa
'sound unstressed' is the definition.
(I've seen this before)
'chief visiting was abused' is the wordplay.
'chief' becomes 'ch' (I've seen this in other clues**).
'visiting' is an insertion indicator.
'abused' is an anagram indicator.
'was' anagrammed gives 'swa'.
'ch' going inside 'swa' is 'SCHWA'.
'after' acts as a link.
(Other definitions for schwa that I've seen before include "sound unstressed" , "Stress-free sound" , "Uh' sound" , "Indistinct sound" , "Unstressed vowel (phonetics)" .)