Mean setter acted in wrong (8)
I believe the answer is:
indicate
'mean' is the definition.
(thesaurus)
'setter acted in wrong' is the wordplay.
'setter' becomes 'i' (the person that set the crossword).
'wrong' indicates an anagram.
'acted'+'in'='actedin'
'actedin' anagrammed gives 'ndicate'.
'i'+'ndicate'='INDICATE'
(Other definitions for indicate that I've seen before include "Point out or give some notion of" , "Point out, make known" , "Make a signal" , "Point out, suggest" , "(Give a) signal" .)