Plain people (5)
I believe the answer is:
'plain people' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for amish that I've seen before include "Member of a sect" , "Relating to a strict American sect" , "Relating to a strict US Mennonite sect" , "US sectarians" , "Of US Mennonites" .)