Nasty plan (4)
I believe the answer is:
mean
I believe this clue is a double definition.
'nasty' is the first definition.
('mean' can be similar in meaning to 'nasty')
'plan' is the second definition.
(thesaurus)
(Other definitions for mean that I've seen before include "Base; tight" , "Intend, plan" , "Intend - shabby" , "design" , "Average; inferior; intend" .)