Yes, EXPOUNDING is a Scrabble word!

Letter values in Scrabble US: E 1, X 8, P 3, O 1, U 1, N 1, D 2, I 1, N 1, G 2.

Is it a valid word?

EXPOUNDING is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a systematic interpretation or explanation (usually written) of a specific topic

add details, as to an account or idea; clarify the meaning of and discourse in a learned way, usually in writing
state

Also: flesh out, set forth.