At all costs dismiss call as coming from youngsters (4)
I believe the answer is:
tots
'youngsters' is the definition.
(tot is a kind of youngster)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tots that I've seen before include "Little children" , "Finds the sum" , "Little drinks" , "Babies" , "Party drinks" .)