Arrangement of unknown number starting with sound of tiny feet (7)
I believe the answer is:
pattern
'arrangement' is the definition.
(thesaurus)
'unknown number starting with sound of tiny feet' is the wordplay.
'unknown number' becomes 'n'.
'starting with' says to put letters next to each other.
'sound of tiny feet' becomes 'patter'.
'n' after 'patter' is 'PATTERN'.
'of' is the link.
(Other definitions for pattern that I've seen before include "Stencil" , "Ornamentation" , "A guide for making something" , "Decorative design; model" , "Arrangement of repeated parts of design" .)