Rationale for decapitation: acts of betrayal (7)
I believe the answer is:
reasons
'rationale' is the definition.
'for decapitation acts of betrayal' is the wordplay.
'for decapitation' means to remove the first letter.
'acts of betrayal' becomes 'treasons' (I've seen this before).
'treasons' with its first letter taken away is 'REASONS'.
(Other definitions for reasons that I've seen before include "excuses" , "Logically deduces" , "Rationalises" , "Ness oar (anag.)" , "Infers logically" .)