Swiftness with which the thing eats into plant on allotment? (8)
I believe the answer is:
celerity
'swiftness' is the definition.
(celerity means swiftness)
'the thing eats into plant on allotment?' is the wordplay.
'the thing' becomes 'it'.
'eats into' indicates putting letters inside.
'plant on allotment?' becomes 'celery' (celery might be grown on an allotment).
'it' put within 'celery' is 'CELERITY'.
'with which' acts as a link.
(Other definitions for celerity that I've seen before include "Haste" , "Expedition" , "Swiftness of movement" , "Quickness" , "showing a good turn of speed" .)