Talk foolishly about one copyright infringer (6)
I believe the answer is:
'copyright infringer' is the definition.
(as in film piracy)
'talk foolishly about one' is the wordplay.
'talk foolishly' becomes 'prate' (to prate is to talk nonsense).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'prate' placed around 'i' is 'PIRATE'.
(Other definitions for pirate that I've seen before include "Corsair" , "sailor" , "Freebooter" , "Maritime robber" , "Rover" .)