WC has to be fixed and sound (5)
I believe the answer is:
schwa
'sound' is the definition.
(schwa is a kind of sound)
'wc has to be fixed' is the wordplay.
'to be fixed' indicates anagramming the letters.
'wc'+'has'='wchas'
'wchas' with letters rearranged gives 'SCHWA'.
'and' acts as a link.
(Other definitions for schwa that I've seen before include "Unstressed vowel (phonetics)" , "Stress-free sound" , "sound unstressed" , "Indistinct sound" , "Uh' sound" .)