Youngsters took test regularly (4)
I believe the answer is:
tots
'youngsters' is the definition.
(tot is a kind of youngster)
'took test regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'took'+'test'='tooktest'
The alternating letters of 'tooktest' are 'TOTS'.
(Other definitions for tots that I've seen before include "little people" , "Party drinks" , "Adds up for the youngsters" , "Adds up like small children" , "Little drinks" .)