Systematically showing plant part (4)
I believe the answer is:
stem
'plant part' is the definition.
(I know that stem is a type of plant organ)
'systematically showing' is the wordplay.
'showing' indicates the answer is hidden within the clue (the answer is shown in the clue).
'STEM' can be found hidden inside 'systematically'.
(Other definitions for stem that I've seen before include "part of vegetable?" , "race" , "root" , "Staunch - stalk" , "Stalk; stop (flow)" .)