To scatter seed around the end of autumn is stupid! (5)
I believe the answer is:
dense
'is stupid' is the definition.
(similar in meaning)
'to scatter seed around the end of autumn' is the wordplay.
'to scatter' indicates anagramming the letters.
'around' is an insertion indicator.
'the end of' says to take the final letters.
The final letter of 'autumn' is 'n'.
'seed' is an anagram of 'dese'.
'dese' placed around 'n' is 'DENSE'.
(Other definitions for dense that I've seen before include "Thick; stupid" , "Tightly packed - stupid" , "Closely compacted" , "Needs (anag) -- thick" , "Close - compact - stupid" .)