Turncoat's first motive for crime (7)
I believe the answer is:
treason
'crime' is the definition.
(treason is a kind of crime)
'turncoat's first motive' is the wordplay.
'first' indicates taking the first letters.
'motive' becomes 'reason' (reason is a kind of motive).
The first letter of 'turncoats' is 't'.
't'+'reason'='TREASON'
'for' is the link.
(Other definitions for treason that I've seen before include "subversion" , "Disloyalty (to one's country)" , "Crime agains one's country" , "Crime of treachery against the state" , "Criminal attempt to overthrow a government" .)