Sign a posh wig should be returned by end of day (6)
I believe the answer is:
augury
'sign' is the definition.
(an augury is a sign or omen)
'a posh wig should be returned by end of day' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'wig' becomes 'rug' (rug is a slang term for a wig).
'should be returned' says the letters should be written backwards.
'by' means one lot of letters go next to another.
'end of' suggests the final letters.
The final letter of 'day' is 'y'.
'rug' back-to-front is 'gur'.
'a'+'u'+'gur'+'y'='AUGURY'
(Other definitions for augury that I've seen before include "Portent or harbinger" , "Divination, prediction" , "Prophesy" , "Omen or its interpretation" , "Sign or omen" .)