Talk at length about right dress (4)
I believe the answer is:
garb
'dress' is the definition.
('garb' can be a synonym of 'dress')
'talk at length about right' is the wordplay.
'talk at length' becomes 'gab' (I've seen this before).
'about' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'gab' enclosing 'r' is 'GARB'.
(Other definitions for garb that I've seen before include "Outfit" , "Clothe" , "Style of clothing" , "Covering" , "Kit" .)