Youth leaders in London and Dublin (3)
I believe the answer is:
lad
'youth' is the definition.
(lad means a young man)
'leaders in london and dublin' is the wordplay.
'leaders in' says to take the initial letters.
The initial letters of 'london and dublin' is 'LAD'.
(Other definitions for lad that I've seen before include "Boy, young man" , "Boy, familiar" , "Chap, fellow" , "(Housman's Shropshire) boy" , "Stable employee" .)