7 down
Capacity
remains
for
out-of-this-world
23
(5,4)
Capacity remains for out-of-this-world 23 (5,4)
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The definition is at the end of the clue.
How the clue works
Look at out-of-this-world 23 — that's your definition, where 23 is a cross-reference to another clue in the puzzle, and the combination points you to SPACE DUST as that cosmic, extraterrestrial matter. The wordplay is a charade: capacity gives you SPACE, and remains gives you DUST (as in mortal remains), and you just slot them together.