Undercover soldiers in splinter group (6)
I believe the answer is:
secret
'undercover' is the definition.
(I know that undercover can be written as secret)
'soldiers in splinter group' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'in' indicates putting letters inside.
'splinter group' becomes 'sect' (I've seen this before).
're' put within 'sect' is 'SECRET'.
(Other definitions for secret that I've seen before include "It's not to be told" , "Kept hidden from others" , "'Hidden, covert (6)'" , "Esoteric" , "avoiding exposure" .)