Enemy agents quietly dropped next to a shrub (7)
I believe the answer is:
syringa
'shrub' is the definition.
(type of shrub)
'enemy agents quietly dropped next to a' is the wordplay.
'enemy agents' becomes 'spy ring' (a ring of enemy spies).
'quietly' becomes 'p' (p for piano and quiet).
'dropped' indicates named letters should be taken away.
'next to' is a charade indicator (letters next to each other).
'spyring' with 'p' taken out is 'syring'.
'syring'+'a'='SYRINGA'
(Other definitions for syringa that I've seen before include "Genus of flowering plants" , "Lilac genus" , "Flowering shrub" , "Is angry (anag.)" , "Mock-orange plant" .)