Slicing tops off salad plant is burden (6)
I believe the answer is:
cumber
'burden' is the definition.
(thesaurus)
'slicing tops off salad plant is' is the wordplay.
I cannot quite understand how this works, but
'is' could be 'be' and 'be' is found in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for cumber that I've seen before include "Get in the way of" , "Impede" , "Hinder" .)