Seldom undercooked, large youth's starter (6)
I believe the answer is:
rarely
'seldom' is the definition.
(I know that seldom can be written as rarely)
'undercooked large youth's starter' is the wordplay.
'undercooked' becomes 'rare' (I've seen this in another clue).
'starter' indicates taking the first letters.
The initial letters of 'large youths' is 'ly'.
'rare'+'ly'='RARELY'
(Other definitions for rarely that I've seen before include "very well" , "Not very often, seldom in fact" , "Not done or occurring very often" , "Seldom, not frequently" , "Gas used in fluorescent signs" .)