To arrange unit on left (5)
I believe the answer is:
until
'to' is the definition.
(thesaurus)
'arrange unit on left' is the wordplay.
'arrange' is an anagram indicator.
'on' is a charade indicator (letters next to each other).
'left' becomes 'L' (common abbreviation).
'unit' anagrammed gives 'unti'.
'unti'+'l'='UNTIL'
(Other definitions for until that I've seen before include "Before a given time" , "As late as" , "As far as" , "Unlit (anag.)" , "Up to that moment" .)