Some red geraniums in the border (4)
I believe the answer is:
edge
'the border' is the definition.
('edge' can be a synonym of 'border')
'some red geraniums in' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'EDGE' can be found hidden inside 'red geraniums in'.
(Other definitions for edge that I've seen before include "incisiveness" , "Demarcation" , "Advantage (informal)" , "Border, fringe" , "Lip; advantage" .)