Plant expert cut on top (5)
I believe the answer is:
aster
'plant' is the definition.
(aster is a kind of plant)
'expert cut on top' is the wordplay.
'expert' becomes 'taster' (taster is a kind of expert).
'cut on top' means to remove the first letter.
'taster' with its first letter taken off is 'ASTER'.
(Other definitions for aster that I've seen before include "Stare oddly at the flower" , "Stare at pretty flower" , "Daisy-like garden flower" , "Plant like a Michaelmas daisy" , "Showy garden flower" .)