Big or little, it's to note (3)
I believe the answer is:
toe
'big or little' is the definition.
(I've seen this in another clue)
'to note' is the wordplay.
'note' becomes 'e' (musical note).
'to'+'e'='TOE'
'it's' acts as a link.
(Other definitions for toe that I've seen before include "One with sock on" , "Extremity of the body" , "End of foot" , "Small member" , "One of the lower digits" .)