To manoeuvre slily is stupid (5)
I believe the answer is:
silly
'is stupid' is the definition.
(I've seen this before)
'to manoeuvre slily' is the wordplay.
'to manoeuvre' is an anagram indicator.
'slily' is an anagram of 'SILLY'.
(Other definitions for silly that I've seen before include "Absurd" , "Potty or barmy" , "Inane; very close (fielder)" , "Daft" , "Not serious" .)