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' is a charade indicator (letters next to each other).
'day' becomes 'd' (abbreviation for day).
'four' becomes 'IV' (Roman numerals for four).
'out' is an anagram indicator (out can mean wrong or inaccurate).
'set' is an anagram of '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" .)