On day four set out, or take off (6)
I believe the answer is:
divest
'take off' is the definition.
(thesaurus)
'on day four set out' is the wordplay.
'on' means one lot of letters go next to another.
'day' becomes 'd' (abbreviation for day).
'four' becomes 'IV' (roman numeral).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'set' anagrammed gives 'est'.
'd' next to 'iv' is 'div'.
'div'+'est'='DIVEST'
'or' acts as a link.
(Other definitions for divest that I've seen before include "Strip off clothes, take away" , "Deprive, strip" , "Strip, denude" , "Relieve (oneself of garment)" , "Take off, say clothing" .)