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' (Roman numeral for 500).
'possibly' indicates an anagram.
'seen' with letters rearranged gives 'ense'.
'd'+'ense'='DENSE'
'as' is the 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" .)