Each 3-digit answer has a different lead digit. This lead digit is replaced by the lead digit from another 3-digit answer before entry, the new lead digits forming a permutation of the old. Clues references are to grid entries. No answer starts with a zero; functions of a number such as multiples, anagrams differ from the number itself.
Across
3 8a-1d
6 square - 2d
8 anagram(1d)
10 square - 12a
12 see 10a
13 square
Down
1 triangular
2 multiple(6a)
4 multiple(13a)
5 multiple(9d)
7 3a - square
9 see 5d
11 square root(8a)