Talks of having one infiltrate the thieves (7)
I believe the answer is:
pirates
'the thieves' is the definition.
(pirate is a kind of thief)
'talks of having one infiltrate' is the wordplay.
'talks' becomes 'prates' (prating is a kind of talking).
'of having' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'one infiltrate' is an insertion indicator.
'prates' enclosing 'i' is 'PIRATES'.
Can you help me to learn more?
(Other definitions for pirates that I've seen before include "Bad sailors" , "Buccaneers" , "Robbers at sea" , "Chatters, talks foolishly" , "Robbers on the high seas" .)