Craftsmen getting on in service (6)
I believe the answer is:
masons
'craftsmen' is the definition.
(mason is a kind of craftsman)
'on in service' is the wordplay.
'in' is an insertion indicator.
'service' becomes 'mass' (religious service).
'on' inserted within 'mass' is 'MASONS'.
'getting' acts as a link.
(Other definitions for masons that I've seen before include "They work with stone and brick" , "Workers with stone - may be Free" , "Craftsmen working in stone" , "Stoneworkers - may be free" .)