A strange plant with arrow-shaped leaves (4)
I believe the answer is:
arum
'plant with arrow-shaped leaves' is the definition.
(I've seen this in another clue)
'a strange' is the wordplay.
'strange' becomes 'rum' (synonyms).
'a'+'rum'='ARUM'
(Other definitions for arum that I've seen before include "Type of 1 Across" , "A sort of lily." , "plant with a white spathe" , "Plant with small flowers and arrow-shaped leaves - kind of lily" , "Perennial" .)