Slow yankee replacing final third of blue box (5)
I believe the answer is:
tardy
'slow' is the definition.
(tardy means running late)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for tardy that I've seen before include "Belated" , "Late; sluggish" , "Dilatory" , "Late, delayed" , "Behindhand" .)