It's within me to be as a tiny child (4)
I believe the answer is:
mite
'child' is the definition.
(I've seen this in other clues)
'it's within me to be as a tiny' is the wordplay.
I cannot really understand how this works, but
'me' could be 'mi' (both are spellings of the musical note) and 'mi' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for mite that I've seen before include "Minute creature" , "Old piece" , "The widow's small donation" , "Little chap" , "Tiny arachnid, person or amount of money" .)