The former, who should have been here by now? (4)
I believe the answer is:
late
'the former' is the definition.
(late can mean former or previous)
'should have been here by now?' is the wordplay.
I cannot quite see how this works, but
'by' could be 'at' and 'at' is located in the answer.
This explanation may well be incorrect...
'who' is the link.
Can you help me to learn more?
(Other definitions for late that I've seen before include "After the usual time" , "Dead or not punctual" , "Unpunctual or dead (4)" , "Behindhand" , "Behind time; deceased" .)