Boys — 50 — are terribly sad (4)
I believe the answer is:
lads
'boys' is the definition.
(lad is a kind of boy)
'50 are terribly sad' is the wordplay.
'50' becomes 'l' (roman numeral).
'are terribly' is an anagram indicator.
'sad' anagrammed gives 'ads'.
'l'+'ads'='LADS'
(Other definitions for lads that I've seen before include "Youngsters" , "Young men" , "Boys, young fellows" , "Fellows, chaps" , "Fellows, blokes" .)