I think it's something like
Fishy to begin with,
A little unreasonably rounded off.
Fishes have fins; the word FIN rounded off becomes IN. This rounding off is 'unreasonable' because it's unreasonable to expect solvers to find it without even the right first letter!
Six faces in the middle,
A solid with six faces is a CUBE, which we find in the middle of the word.
But needs to be closed a million times.
The final part of the word is ATE. In all the meals you ate in your life, your mouth must have needed to be closed at least a million times!
Stumped by this riddle?
Then get off your computer and go live a little!
Thanks for the advice - I'll go and do that right now ;-)