Queen's brought back medicinal plant (5)
I believe the answer is:
senna
'medicinal plant' is the definition.
(I've seen this in another clue)
'queen's brought back' is the wordplay.
'queen's' becomes 'anne's' (Queen Anne, 18th-century queen).
'brought back' says the letters should be written backwards.
'annes' in reverse letter order is 'SENNA'.
(Other definitions for senna that I've seen before include "Cassia tree, laxative" , "Plant with medicinal pods" , "Natural laxative" , "He drove fast" , "It produces pods" .)