Youngsters left, terribly sad (4)
I believe the answer is:
lads
'youngsters' is the definition.
(I've seen this before)
'left terribly sad' is the wordplay.
'left' becomes 'L' (common abbreviation).
'terribly' indicates an anagram.
'sad' with letters rearranged gives 'ads'.
'l'+'ads'='LADS'
(Other definitions for lads that I've seen before include "Fellows, chaps" , "Fellows, blokes" , "Young men" , "Youngsters" , "Boys, young fellows" .)