A note, or 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'
'or' is the link.
(Other definitions for minim that I've seen before include "Note equal to two crochets" , "Two crotchets in one" , "A note in music, half as long as a semi-breve- it's a palindrome" , "Feature of a score" , "Beat" .)