Are you sure you have the letters right? A 6 letter word for a candy treat starting with 's' is 'sweets'.
There are only two letters starting with L that I can think of; the Letter of the alphabet, which is "L" and the Word "Letter." As "Letter" is longer than "L" I can only assume that the word "Letter" is the longest letter starting with L. lymphoblastic leukemia
Words starting with L from the letters in 'usable' are:lab, labslea, leaslube, lubes