Dressed in right old sack (5)
I believe the answer is:
robed
'dressed' is the definition.
('robed' can be a synonym of 'dressed')
'right old sack' is the wordplay.
'right' becomes 'r' (common abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sack' becomes 'bed' ('in the sack' means 'in bed').
'r'+'o'+'bed'='ROBED'
'in' acts as a link.
(Other definitions for robed that I've seen before include "Attired" , "Wearing judge's garments" , "Dressed" , "In a long garment" .)