Democrat joining secret group gives service (6,3)
'service' is the definition.
(dinner set is a kind of service)
'democrat joining secret group' is the wordplay.
'democrat' becomes 'd'.
'joining' says to put letters next to each other.
'secret' becomes 'inner' (I've seen this before).
'group' becomes 'set'.
'gives' is the link.
(Other definitions for dinner set that I've seen before include "crockery", "Collection of plates and dishes", "Plates etc", "might be taken out for a meal", "Complete service of plates and dishes for a meal".)