Thief's initial motive for crime (7)
I believe the answer is:
treason
'crime' is the definition.
(treason is a kind of crime)
'thief's initial motive' is the wordplay.
'initial' suggests taking the first letters.
'motive' becomes 'reason' (reason is a kind of motive).
The initial letter of 'thiefs' is 't'.
't'+'reason'='TREASON'
'for' is the link.
(Other definitions for treason that I've seen before include "LEse-majestE" , "Lese-majesty" , "Senator (anag)" , "Stare on (anag.)" , "Mutiny" .)