Thomas's initial motive for betrayal (7)
I believe the answer is:
treason
'betrayal' is the definition.
('treason' can be a synonym of 'betrayal')
'thomas's initial motive' is the wordplay.
'initial' indicates taking the first letters.
'motive' becomes 'reason' (reason is a kind of motive).
The initial letter of 'thomass' is 't'.
't'+'reason'='TREASON'
'for' is the link.
(Other definitions for treason that I've seen before include "rebellion" , "Disloyalty (to one's country)" , "LEse-majestE" , "Stare on (anag.)" , "One's art? (anag)" .)