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