The titular head has a motive for betrayal (7)
I believe the answer is:
treason
'for betrayal' is the definition.
(synonyms)
'the titular head has a motive' is the wordplay.
'head' indicates taking the first letters.
'has' means one lot of letters go next to another.
'a motive' becomes 'reason' (reason is a kind of motive).
The initial letter of 'titular' is 't'.
't'+'reason'='TREASON'
(Other definitions for treason that I've seen before include "Crime of national betrayal" , "Serious crime" , "LEse-majestE" , "subversion" , "Betrayal of one's nation" .)