Leaders of Caerphilly young men's rugby union in Wales (5)
I believe the answer is:
cymru
'wales' is the definition.
('Cymru' can be a synonym of 'wales')
'leaders of caerphilly young men's rugby union' is the wordplay.
'leaders of' indicates taking the first letters.
'rugby union' becomes 'ru'.
The first letters of 'caerphilly young mens' is 'cym'.
'cym'+'ru'='CYMRU'
'in' acts as a link.
(Other definitions for cymru that I've seen before include "European country" , "Welsh name for Wales" , "Part of UK" .)