Investigates frontless dresses (5)
I believe the answer is:
robes
'dresses' is the definition.
(robing is a kind of dressing)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for robes that I've seen before include "trappings of office?" , "Official vestments" , "Attire" , "Gowns" , "Sober (anag)" .)