Plant used in magic act, usually (6)
I believe the answer is:
cactus
'plant' is the definition.
(cactus is a kind of plant)
'in magic act usually' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CACTUS' is hidden within 'magic act usually'.
'used' acts as a link.
(Other definitions for cactus that I've seen before include "Spiny, succulent plant" , "Thorny desert plant" , "Spiney plant of arid regions" , "Spiney plant of dry regions" , "Desert flower" .)