The word "trainers" uses 8 letters: A E I N R R S T.
Words formed by adding one letter before or after trainers (in bold), or to aeinrrst in any order:
d - instarred e - retainers ternaries g - arresting serrating o - serration p - pretrains terrapins transpire s - restrains strainers t - restraint retirants v - veratrins