Extreme disloyalty revealed by letter concerning a male child (7)
I believe the answer is:
treason
'extreme disloyalty' is the definition.
'treason' can be an answer for 'disloyalty' (treason is a kind of disloyalty). I'm not sure about the 'extreme' bit.
'letter concerning a male child' is the wordplay.
'letter' becomes 't'.
'concerning' becomes 're' (both can mean on a particular topic).
'male child' becomes 'son' (I've seen this in another clue).
't'+'re'+'a'+'son'='TREASON'
'revealed by' is the link.
Can you help me to learn more?
(Other definitions for treason that I've seen before include "One's art? (anag)" , "subversion" , "Betrayal of the state" , "Betrayal (of one's country)" , "Criminal attempt to overthrow a government" .)