Tons with motive for betrayal (7)
I believe the answer is:
treason
'for betrayal' is the definition.
(synonyms)
'tons with motive' is the wordplay.
'tons' becomes 't'.
'with' says to put letters next to each other.
'motive' becomes 'reason' (reason is a kind of motive).
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "Mutiny" , "Gunpowder Plot, say" , "Senator (anag)" , "rebellion" , "attempting to overthrow the king" .)