Many possibly seen as stupid (5)
I believe the answer is:
dense
'stupid' is the definition.
(similar in meaning)
'many possibly seen' is the wordplay.
'many' becomes 'd' ().
'possibly' indicates an anagram.
'seen' anagrammed gives 'ense'.
'd'+'ense'='DENSE'
'as' acts as a link.
(Other definitions for dense that I've seen before include "Close knit" , "Stupid; compacted" , "Thick, compact or crowded" , "Close - compact - stupid" , "Closely crowded together" .)