Say you hear a cat (8)
I believe the answer is:
announce
'say you' is the definition.
'announce' can be an answer for 'say' (I've seen this before). I am not certain of the 'you' bit.
'hear a cat' is the wordplay.
'hear' becomes 'ann' (I can't explain this - if you can you should give a lot more credence to this answer).
'a cat' becomes 'ounce' (ounce is a kind of cat).
'ann'+'ounce'='ANNOUNCE'
Can you help me to learn more?
(Other definitions for announce that I've seen before include "Give public notice to" , "Make known publicly with no nuance" , "Make known publicly or to an audience" , "Make known officially" , "Formally declare" .)