Fashionably old-fashioned interior decor, terribly overlooked (5)
I believe the answer is:
'fashionably old-fashioned' is the definition.
(I've seen this in another clue)
'interior decor terribly overlooked' is the wordplay.
'interior' says the answer is hidden in the clue (I've seen 'interior of' mean this).
'overlooked' says the letters should be written backwards.
'orter' is hidden in the letters of 'decor terribly'.
'orter' backwards is 'RETRO'.
(Other definitions for retro that I've seen before include "Predatory fish" , "Fashion recalling the past" , "bit of nostalgia" , "Imitating a style from the recent past" , "sort of rocket" .)