Undercover soldiers to infiltrate extremist group (6)
I believe the answer is:
secret
'undercover' is the definition.
('secret' can be a synonym of 'undercover')
'soldiers to infiltrate extremist group' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'to infiltrate' is an insertion indicator.
'extremist group' becomes 'sect'.
're' put into 'sect' is 'SECRET'.
(Other definitions for secret that I've seen before include "hushed prayer" , "Private; classified" , "Not known by others" , "Inner" , "Confidential (information)" .)