One barely old enough to add up? (3)
I believe the answer is:
tot
'to add up?' is the definition.
('tot' can be a synonym of 'add up')
'one barely old enough' is the wordplay.
'one' becomes 'unitary' (synonyms).
'barely' means to look at the middle letters (the word is bare of its external letters).
'old enough' becomes 'ot' (I can't justify this - if you can you should believe this answer much more).
The middle letter of 'unitary' is 't'.
't'+'ot'='TOT'
Can you help me to learn more?
(Other definitions for tot that I've seen before include "Small baby" , "quick drink" , "Sum for the little one" , "Small drink of spirits" , "Dram; small child" .)