Long garment gnome shortened (4)
I believe the answer is:
maxi
'long garment' is the definition.
(as opposed to a mini skirt)
'gnome shortened' is the wordplay.
'gnome' becomes 'maxim' (both can mean a saying).
'shortened' means to remove the last letter.
'maxim' with its last letter taken away is 'MAXI'.
(Other definitions for maxi that I've seen before include "Full-length skirt" , "Relatively long skirt" , "Long skirt of coat" , "full-length garment" , "what garment is not revealing?" .)