First to talk about a boy's treachery (7)
I believe the answer is:
treason
'treachery' is the definition.
(I know that treachery can be written as treason)
'first to talk about a boy's' is the wordplay.
'first to' becomes 't' (rst letter of 'to').
'talk about' becomes 'rea' (I can't justify this - if you can you should give a lot more credence to this answer).
'a boy' becomes 'son' (male child).
't'+'rea'+'son'='TREASON'
Can you help me to learn more?
(Other definitions for treason that I've seen before include "One rats (anag)" , "Act of betrayal of a government" , "Senator (anag)" , "Serious crime" , "Lese-majesty" .)