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 am not sure about this - if you are sure you should believe this answer much more).
'and' says to put letters next to each other.
'it' becomes 'sa' (old-fashioned abbreviation for sex appeal).
'y' 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" .)