Thug's initial motive for serious crime (7)
I believe the answer is:
treason
'serious crime' is the definition.
(treason is a crime)
'thug's initial motive' is the wordplay.
'initial' suggests taking the first letters.
'motive' becomes 'reason' (reason is a kind of motive).
The first letter of 'thugs' is 't'.
't'+'reason'='TREASON'
'for' is the link.
(Other definitions for treason that I've seen before include "Crime of treachery against the state" , "Disloyalty (to one's country)" , "subversion" , "Criminal attempt to overthrow a government" , "Lese-majesty" .)