The first explanation of treachery (7)
I believe the answer is:
treason
'treachery' is the definition.
(I know that treachery can be written as treason)
'the first explanation' is the wordplay.
'the first' becomes 't' (1st letter of 'the').
'explanation' becomes 'reason' (reason is a kind of explanation).
't'+'reason'='TREASON'
'of' acts as a link.
(Other definitions for treason that I've seen before include "Deliberate act of betrayal" , "Offence against state" , "Betraying one's country" , "Gunpowder Plot, say" , "Crime of treachery against the state" .)