Adult feeding toddler perfectly (2,1,1)
I believe the answer is:
to a t
'perfectly' is the definition.
('to a T' or 'to a tee' can mean perfectly)
'adult feeding toddler' is the wordplay.
'adult' becomes 'a' (abbreviation).
'feeding' indicates putting letters inside (one word is fed by another set of letters).
'toddler' becomes 'tot' (synonyms).
'a' placed within 'tot' is 'TO A T'.
(Other definitions for to a t that I've seen before include "Exactly right" , "perfectly well" , "In every detail" .)