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Across
1 Triangular one then square. (3) — 136 is a triangular number that is a one followed by a square number (36).
3 Audible German no between tutus, for one square. (5) — 29241 is a square number that sounds like a German no (nein/nine) between tutus (two two) for one (four one).
5 Irreducible ending Morpheus halloumi fix, then Trinity, then mixed up Neo. (3) — 631 is an irreducible number that is the ending of Morpheus halloumi fix (six), the trinity (three), then mixed up Neo (oNe).
Down
1 Inside Fort Worth following unlucky multiple of eleven. (3) — 132 is a multiple of 11 that is inside Fort Worth (two) following unlucky (13).
2 Palindrome two between two clickety-clicks. (5) — 66266 is a palindrome that is a two between two clickety-clicks (66s).
4 Confused Etna honored thundery din became prime. (3) — 131 is a prime that is an anagram of "Etna honored thundery din" (onE hundred and thirty one).