Hear this and it's only a rumour (3)
I believe the answer is:
say
'hear this and it's only a rumour' is the definition.
The definition suggests a singular noun which matches the answer.
'hear this and it's' is the wordplay.
'hear this' becomes 'y' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' means one lot of letters go next to another.
'it' becomes 'sa' (SA can mean sex appeal, as in 'it').
'y' put after 'sa' is 'SAY'.
Can you help me to learn more?
(Other definitions for say that I've seen before include "Utter; for example" , "Verbalise" , "Let us suppose" , "Opportunity to influence < > state" , "Tell" .)