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