Cruel old ruler’s verse on youth (4)
I believe the answer is:
vlad
'cruel old ruler's' is the definition.
(Vlad the Impaler)
'verse on youth' is the wordplay.
'verse' becomes 'v'.
'on' means one lot of letters go next to another.
'youth' becomes 'lad' (I've seen this before).
'v'+'lad'='VLAD'
(Other definitions for vlad that I've seen before include "Infamous impaler" , "__ Dracula, the 15th century Prince of Wallachia, called the Impaler" .)