Small bear enters in one myth’s night-time demons (6)
I believe the answer is:
incubi
'demons' is the definition.
(incubus is a kind of demon)
'small bear enters in one' is the wordplay.
'small bear' becomes 'cub' (I have seen 'Young bear ' mean 'cub' so perhaps 'bear' could also mean 'cub'. I am not sure about the 'small' bit.).
'enters' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'in'+'i'='ini'
'cub' going within 'ini' is 'INCUBI'.
'myth's night-time' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for incubi that I've seen before include "nightmares" , "Demons of the night" .)