Arcane splinter group holding on (6)
I believe the answer is:
secret
'arcane' is the definition.
(thesaurus)
'splinter group holding on' is the wordplay.
'splinter group' becomes 'sect' (I've seen this before).
'holding' means one lot of letters goes inside another.
'on' becomes 're' ('on' can be mean 'with reference to').
'sect' placed around 're' is 'SECRET'.
(Other definitions for secret that I've seen before include "Not known by others" , "Concealed" , "Not to be disclosed" , "Confidential or mysterious" , "Kept under wraps" .)