Quiet group joined in conspiracy (4)
I believe the answer is:
plot
'conspiracy' is the definition.
(a conspiracy is a secret plot)
'quiet group joined' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'group' becomes 'lot' ().
'joined' is a charade indicator (letters next to each other).
'p'+'lot'='PLOT'
'in' is the link.
(Other definitions for plot that I've seen before include "Ground - plan" , "bed" , "part of garden" , "Connive" , "Gunpowder, for one" .)