Stop following extremely silly plan (6)
I believe the answer is:
system
'plan' is the definition.
(system is a kind of plan)
'stop following extremely silly' is the wordplay.
'stop' becomes 'stem' (stemming is a kind of stopping).
'following' says to put letters next to each other.
'extremely' means to remove the middle letters.
'silly' with its centre taken out is 'sy'.
'stem' after 'sy' is 'SYSTEM'.
(Other definitions for system that I've seen before include "A complete methodology" , "Arrangement of units in a whole" , "Arrangement of units that function together" , "Network" , "Method (for betting?)" .)