Plant on a wall pruned and growing upwards (4)
I believe the answer is:
arum
'plant' is the definition.
(arum is a kind of plant)
'wall pruned and growing upwards' is the wordplay.
'wall' becomes 'mural'.
'pruned' means to remove the last letter.
'and growing upwards' says the letters should be written in reverse.
'mural' with its last letter taken away is 'mura'.
'mura' back-to-front is 'ARUM'.
'on a' is the link.
(Other definitions for arum that I've seen before include "genus of plants" , "plant with a white spathe" , "Type of plant, lily perhaps" , "A spirit for a lily" , "Type of 1 Across" .)