A note is not a full sized letter (5)
I believe the answer is:
minim
'a note' is the definition.
(musical note)
'is not a full sized letter' is the wordplay.
I cannot quite see how this works, but
'letter' could be 'n' and 'n' is found in the answer.
'is' could be 'ii' (more than one I) and 'ii' is found in the leftover letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for minim that I've seen before include "It's a downward stroke" , "Palindromic note" , "note or > dose" , "Quarter of a breve" , "Note equal to two crochets" .)