Speed salad plant absorbs it (8)
I believe the answer is:
celerity
'speed' is the definition.
(I've seen this before)
'salad plant absorbs it' is the wordplay.
'salad plant' becomes 'celery' (I've seen this before).
'absorbs' indicates putting letters inside.
'celery' enclosing 'it' is 'CELERITY'.
(Other definitions for celerity that I've seen before include "Dispatch" , "Swiftness of movement" , "Quickness" , "Haste" , "Expedition" .)