The newest is the worst timekeeper (6)
I believe the answer is:
latest
'the newest' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for latest that I've seen before include "In vogue" , "Most recent or tardiest" , "Most recent (news)" , "News update" , "Most up-to-date" .)