Serious-sounding chap (6)
I believe the answer is:
ernest
'chap' is the definition.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for ernest that I've seen before include "character for Wilde" , "- Hemingway, - Shackleton" , "Name beloved of Gwen Fairfax (Wilde)" , "- Hemingway, US author" , "fellow" .)