With cunning, son locates youth leaders twice (5)
I believe the answer is:
slyly
'with cunning' is the definition.
'son locates youth leaders twice' is the wordplay.
'son' becomes 's'.
'leaders' suggests taking the first letters.
'twice' says to double the letters.
The first letters of 'locates youth' is 'ly'.
'ly' doubled is 'lyly'.
's'+'lyly'='SLYLY'
(Other definitions for slyly that I've seen before include "in underhand way" , "In a cunning manner" , "In a stealthy and secret way" , "Craftily, cunningly" .)