Plant it in haste (8)
I believe the answer is:
celerity
'haste' is the definition.
(I've seen this before)
'plant it in' is the wordplay.
'plant' becomes 'celery' (celery is a kind of plant).
'in' is an insertion indicator.
'celery' placed around 'it' is 'CELERITY'.
(Other definitions for celerity that I've seen before include "Pace" , "Expedition" , "showing a good turn of speed" , "Haste" , "Dispatch" .)