Talk one into becoming a thief (6)
I believe the answer is:
pirate
'a thief' is the definition.
(pirates are thieves)
'talk one into' is the wordplay.
'talk' becomes 'prate' (to prate is to talk nonsense).
'one' becomes 'i' (Roman numeral).
'into' means one lot of letters goes inside another.
'prate' going around 'i' is 'PIRATE'.
'becoming' is the link.
(Other definitions for pirate that I've seen before include "Use (another's work) for profit without permission" , "Criminal on the high seas" , "Teach, perhaps" , "Copyright infringer" , "fake" .)