Youngster to jog but not run (3)
I believe the answer is:
tot
'youngster' is the definition.
(I know that tot is a type of youngster)
'jog but not run' is the wordplay.
'jog' becomes 'trot' (synonyms).
'but not' indicates named letters should be taken away.
'run' becomes 'r' (cricket abbreviation).
'trot' with 'r' taken out is 'TOT'.
'to' acts as a link.
(Other definitions for tot that I've seen before include "Small quantity of alcoholic drink" , "Child; measure of spirits" , "Add up like infant" , "quick drink" , "Rum measure" .)