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' suggests taking the first letters.
The first 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" .)