The meta answer is RESPONSIBLE (Hint: Look for an 11-letter adjective).
Five symmetrically placed Across answers had clues with both an upper-case first letter (which, of course!) and an unusually upper-case last letter as well. (In the original print version of this puzzle, those first and last letters of those clues were subtly bolded).
This might have led you to notice that the first and last upper-case letters of those five clues also appear in their own answers, but with a few extra letters in between them.
Before we get to those in-between letters, let's recap those clues:
17A: "OPEN," "SALE!", and "10 items or lesS"
23A: Under typical conditions, the state water's in when temps are higH
39A: Thing that gets "pulled" in order to deceivE
51A: Elastic properties that allow stretched materials to snap back to normaL
63A: Rich coffee cake with a two-color swirL
From top to bottom, the first upper-case letters spelled OUTER and the last upper-case letters spell SHELL. So we've not got an OUTER SHELL... so what's that to do with an 11-letter adjective?
The puzzle's title, "Solid Inner Core," hints that the answer involves the inside of something. Could there be something within that OUTER SHELL?
Yes, that's where those "in-between" letters come in. Here are the answers, with the "OUTER SHELL" letters in square brackets:
17A: ST[O]RE[S]SIGNS
23A: GASEO[U]SP[H]ASE
39A: FAS[T]ON[E]
51A: R[E]SI[L]IENCIES
63A: MA[R]BLE[L]OAF
Recall the puzzle's title, "Solid Inner Core." From top to bottom, the letters at the core of the grid's "OUTER SHELL" letters spell RESPONSIBLE, which is a synonym for "solid" and an 11-letter adjective that satisfies the puzzle's hint.