Princess's climbing plant (5)
I believe the answer is:
senna
'plant' is the definition.
The answer is an example.
'princess's climbing' is the wordplay.
'princess's' becomes 'Anne's' (Princess Anne, daughter of Queen Elizabeth).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'annes' written backwards gives 'SENNA'.
(Other definitions for senna that I've seen before include "purgative's needed" , "Late driver" , "taking this to speed up motion?" , "It produces pods" , "Formula One driver" .)