Lawn weed: cut the whole lot back (8)
I believe the answer is:
prunella
'lawn weed' is the definition.
(prunella is a plant genus)
'cut the whole lot back' is the wordplay.
'cut' becomes 'prune' (synonyms).
'the whole lot' becomes 'all'.
'back' says the letters should be written backwards.
'all' reversed gives 'lla'.
'prune'+'lla'='PRUNELLA'
(Other definitions for prunella that I've seen before include "flower" , "-- Scales, actress" , "Strong silk" , "It reputedly heals everyone" , "climbing plant" .)