Villain in connection with specially trained group (5)
I believe the answer is:
cadre
'specially trained group' is the definition.
(I've seen this in another clue)
'villain in connection with' is the wordplay.
'villain' becomes 'cad' (cad can mean a nasty person).
'in connection with' becomes 're' (regarding).
'cad'+'re'='CADRE'
(Other definitions for cadre that I've seen before include "team" , "Fighting unit" , "Raced (anag) - small central group" , "Nucleus, corps" , "Raced about in military group" .)