During the day, left a trainee to be idle (5)
I believe the answer is:
dally
'be idle' is the definition.
(thesaurus)
'during the day left a trainee' is the wordplay.
'during' is an insertion indicator.
'left' becomes 'l' (common abbreviation).
'a trainee' becomes 'l'.
'l'+'l'='ll'
'day' going around 'll' is 'DALLY'.
'to' is the link.
(Other definitions for dally that I've seen before include "Waste time" , "Trifle" , "Procrastinate" , "Linger" , "Have an affair with" .)