Joint contribution advanced by a group (5)
I believe the answer is:
tenon
'joint' is the definition.
(I've seen this in other clues)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for tenon that I've seen before include "Tongue of wood for joint" , "Mortise extension" , "Mortise counterpart" , "Projection (joins with mortise)" , "Join" .)