Plump louse eats plant ends, causing blight (4)
I believe the answer is:
pest
'blight' is the definition.
(thesaurus)
'plump louse eats plant ends' is the wordplay.
'ends' suggests the final letters.
The last letters of 'plump louse eats plant' are 'PEST'.
'causing' acts as a link.
(Other definitions for pest that I've seen before include "Destructive creature" , "Plague" , "Destructive insect or nuisance" , "Constant nuisance" , "Troublesome man or animal" .)