Needs reform, being stupid (5)
I believe the answer is:
dense
'stupid' is the definition.
(similar in meaning)
'needs reform' is the wordplay.
'reform' indicates anagramming the letters (letters in a new form).
'needs' with letters rearranged gives 'DENSE'.
'being' is the link.
(Other definitions for dense that I've seen before include "Close-knit" , "Not very bright" , "Close-packed" , "Thick and tightly-packed" , "Thick; stupid" .)