Presumably he had a good reason for being late (6)
I believe the answer is:
martyr
'presumably he had a good reason for being late' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'presumably he' is the wordplay.
I cannot really understand how this works, but
'he' could be 'marty' and 'marty' is present in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for martyr that I've seen before include "Persecution victim" , "Sufferer for faith" , "One sacrificed to cause" , "One suffering or dying for a cause" , "One dying for their faith" .)