Spectators, mostly very wise, somehow (7)
I believe the answer is:
viewers
'spectators' is the definition.
(people viewing or spectating something)
'mostly very wise somehow' is the wordplay.
'mostly' means to remove the last letter.
'somehow' is an anagram indicator.
'very' with its last letter removed is 'ver'.
'ver'+'wise'='verwise'
'verwise' anagrammed gives 'VIEWERS'.
(Other definitions for viewers that I've seen before include "spectators" , "TV addicts" .)