Sadly clear to me, he's a tardy person (9)
I believe the answer is:
latecomer
'he's a tardy person' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'sadly clear to me' is the wordplay.
'sadly' is an anagram indicator.
'clear'+'to'+'me'='cleartome'
'cleartome' with letters rearranged gives 'LATECOMER'.
Can you help me to learn more?
(Other definitions for latecomer that I've seen before include "Perhaps one held up" , "Recent or tardy arrival" , "Delayed arrival" , "Recent convert" , "one's a poor timekeeper" .)