One chronicling chap's century, suppressing regressive attitude (9)
I believe the answer is:
historian
'one chronicling' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'chap's century suppressing regressive attitude' is the wordplay.
'chap's' becomes 'his' (belonging to a man).
'century' becomes 'ton' (both can mean one hundred of something).
'suppressing' is an insertion indicator.
'regressive' is a reversal indicator (to regress is to go backwards).
'attitude' becomes 'air' (I've seen this before).
'air' backwards is 'ria'.
'ton' placed around 'ria' is 'torian'.
'his'+'torian'='HISTORIAN'
Can you help me to learn more?
(Other definitions for historian that I've seen before include "Writer about the past" , "student of the past" , "Past master" , "chronicler" , "One sort of academic" .)