A variable rate — it's a little number (7)
I believe the answer is:
arietta
'a little number' is the definition.
(short piece of music or musical number)
'a variable rate it's' is the wordplay.
'variable' indicates an anagram.
'rate'+'it'='rateit'
'rateit' is an anagram of 'rietta'.
'a'+'rietta'='ARIETTA'
(Other definitions for arietta that I've seen before include "Short operatic song" , "Item by 6 [VERDI]" , "number from opera" , "Little song" , "Short operatic number" .)