Gross ranter's somehow seen as wrongdoer (12)
I believe the answer is:
transgressor
'wrongdoer' is the definition.
(transgressor is a kind of wrongdoer)
'gross ranter's somehow' is the wordplay.
'somehow' is an anagram indicator.
'gross'+'ranters'='grossranters'
'grossranters' is an anagram of 'TRANSGRESSOR'.
'seen as' is the link.
(Other definitions for transgressor that I've seen before include "One who oversteps a mark" , "Lawbreaker - grass snorter (anag)" , "Offender; sinner" , "One who oversteps the limit" .)