Backache after beginning to prune plant (8)
I believe the answer is:
plumbago
'plant' is the definition.
(plumbago is a kind of plant)
'backache after beginning to prune' is the wordplay.
'backache' becomes 'lumbago' (lumbago is a kind of backache).
'after' says to put letters next to each other.
'beginning to' says to take the initial letters.
The first letter of 'prune' is 'p'.
'lumbago' after 'p' is 'PLUMBAGO'.
(Other definitions for plumbago that I've seen before include "Word for graphite" , "lead" , "Blue-flowered plant" , "Plant type that includes sea pinks" , "Shrub with blue flowers" .)