Shorten, as a skirt (5)
'shorten as a skirt' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hem that I've seen before include "Finished edge of a piece of cloth" , "Part of skirt" , "Double-backed border of a garment" , "Sewn border" , "try to attract attention" .)