A note is a little letter (5)
I believe the answer is:
minim
'a note' is the definition.
(type of musical note)
'little letter' is the wordplay.
'little' becomes 'mini'.
'letter' becomes 'm' ().
'mini'+'m'='MINIM'
'is a' is the link.
(Other definitions for minim that I've seen before include "It's a downward stroke" , "Half-note" , "Musical note - small amount of liquid" , "Two crotchets in one" , "Note with value of half a semibreve" .)