Members of a secret society getting on in service (6)
I believe the answer is:
masons
'members of a secret society' is the definition.
'Masons' can be an answer for 'members' (Mason is an example). I am unsure of the 'of a secret society' bit.
'on in service' is the wordplay.
'in' is an insertion indicator.
'service' becomes 'mass' (church service).
'on' put into 'mass' is 'MASONS'.
'getting' acts as a link.
Can you help me to learn more?
(Other definitions for masons that I've seen before include "They work with stone and brick" , "Stoneworkers - may be free" , "Workers with stone - may be Free" , "Craftsmen working in stone" .)