Cause betrayal -- lose head (6)
I believe the answer is:
reason
'cause' is the definition.
(I know that cause can be written as reason)
'betrayal lose head' is the wordplay.
'betrayal' becomes 'treason' (synonyms).
'lose head' means to remove the first letter (lose (remove) the head (start) of the word).
'treason' with its initial letter removed is 'REASON'.
(Other definitions for reason that I've seen before include "Motive; argue" , "Justification (for action)" , "Cause; think logically" , "brain operations" , "Explanation or motive" .)