Salad plant is stuffed with it for speed (8)
I believe the answer is:
celerity
'speed' is the definition.
(I've seen this before)
'salad plant is stuffed with it' is the wordplay.
'salad plant' becomes 'celery' (I've seen this before).
'is stuffed with' means one lot of letters goes inside another (I've seen this in other clues).
'celery' going around 'it' is 'CELERITY'.
'for' is the link.
(Other definitions for celerity that I've seen before include "Quickness" , "Haste" , "Pace" , "Dispatch" , "Swiftness of movement" .)