Ancient noble youngsters face (5)
I believe the answer is:
early
'ancient' is the definition.
(I've seen this in another clue)
'noble youngsters face' is the wordplay.
'noble' becomes 'earl' (earl is a kind of noble).
'face' suggests taking the first letters.
The initial letter of 'youngsters' is 'y'.
'earl'+'y'='EARLY'
(Other definitions for early that I've seen before include "Premature" , "Before due time" , "at start of day" , "In good time" , "Before appointed time" .)