Tree-dweller father observed crossing railway (5)
I believe the answer is:
dryad
This clue might be a double definition.
'tree-dweller' is the first definition.
(type of spirit in Greek myth)
'father observed crossing railway' is the second definition.
The answer and definition can be both people as well as being singular nouns.
Maybe there's an association between them I don't understand?
Can you help me to learn more?
(Other definitions for dryad that I've seen before include "butterfly" , "Nymph of the woods" , "Tree nymph" , "Mythological wood nymph" , "Beautiful young woman" .)