Plant some massed geraniums (5)
I believe the answer is:
sedge
'plant' is the definition.
(sedge is a plant)
'some massed geraniums' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'SEDGE' is hidden within 'massed geraniums'.
(Other definitions for sedge that I've seen before include "Wetland grass-like plant" , "Grasslike marsh plant" , "Grass-like plant growing in wet places" , "Plant growing in swampy ground" , "Marsh-dweller" .)