Desert plant (6)
I believe the answer is:
'desert plant' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cactus that I've seen before include "Prickly sort" , "Spiney plant of arid regions" , "Spiky desert plant" , "Plant found in dry places" , "Spiney plant of dry regions" .)