Further adding to Blanks/Gmans "therein:"
They had: the, there, tin, rein, i, in, then, he, her, here, herein, tern
Add "re" as in "do, re, me, fa, so, la, te, do."
Add "therein" because it must contain itself.
Also, "tern" would be valid even in Scrabble (tm) since it isn't a proper noun. (You would not capitalize it in "I dunno, some kind of tern I guess.")
That makes:
01 therein
02 the
03 there
04 tin
05 rein
06 i
07 in
08 then
09 he
10 her
11 here
12 herein
13 tern
14 re
So were you looking for a word that made exactly 10?