A number needing to change footwear after tango (3,3,3)
I believe the answer is:
tea for two
'a number' is the definition.
(I have seen 'Musical number' mean 'tea for two' so perhaps 'number' could also mean 'tea for two')
'to change footwear after tango' is the wordplay.
'to change' is an anagram indicator.
'after' says to put letters next to each other.
'tango' becomes 'T' (phonetic alphabet: alpha, bravo, charlie etc.).
'footwear' anagrammed gives 'eafortwo'.
'eafortwo' put after 't' is 'TEA FOR TWO'.
'needing' acts as a link.
(Other definitions for tea for two that I've seen before include "Popular 1925 song (for afternoon drinking companions?)" , "Musical number" , "Air" , "Song from musical No, No, Nanette" .)