He possibly enters in earnest, we're told (6)
I believe the answer is:
ernest
'he' is the definition.
(I've seen this before)
'enters in earnest we're told' is the wordplay.
'enters' anagrams to 'ERNEST'.
Though, I'm unsure how the anagram is indicated.
'possibly' is the link.
Can you help me to learn more?
(Other definitions for ernest that I've seen before include "character for Wilde" , "chap" , "- Shackleton, Antarctic explorer" , "Name sounds serious" , "Resent (anag) - man" .)