Declare a cat can be heard (8)
I believe the answer is:
announce
'declare' is the definition.
(synonyms)
'a cat can be heard' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is present in the answer.
'cat' could be 'ounce' (ounce is a kind of cat) and 'ounce' is present in the answer.
A single letter 'n' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for announce that I've seen before include "Make public statement, proclaim" , "Formally declare" , "Make known publicly or to an audience" , "Disclose" , "Declare, proclaim" .)