Perfectly ignoring adult and small child (3)
I believe the answer is:
tot
'small child' is the definition.
(I've seen this before)
'perfectly ignoring adult' is the subsidiary indication.
'perfectly' becomes 'toat' (I've seen this before).
'ignoring' indicates named letters should be taken away.
'adult' becomes 'a'.
'toat' with 'a' removed is 'tot'.
'and' is the link.
(Other definitions for tot that I've seen before include "nipper" , "Get the sum for the toddler" , "Add up like infant" , "An arithmetic babe?" , "Small drink of spirits" .)