Dressing without top of nightie? That’s tawdry (6)
I believe the answer is:
garish
'tawdry' is the definition.
('garish' can be a synonym of 'tawdry')
I cannot understand how the rest of the clue works.
(Other definitions for garish that I've seen before include "Gaudy, over-decorated" , "Bright and showy" , "Tastelessly bright" , "Colourful in a vulgar manner" , "Gaudy, vulgarly bright" .)