I believe that the clue may have a question mark at the end. Chambers shows the suffix '-ist' as indicating advocacy, so an ASS-IST could (fancifully, as the question mark suggests) describe someone or something which is 'for [ie in favour of] idiots', just as a 'royalist group' would favour the royals . The conventional definition is 'Support'.
That doesn't mean that I like the clue, 'cos I don't.