Youth leader ahead of time every twelve months (6)
I believe the answer is:
yearly
'every twelve months' is the definition.
(I've seen this before)
'youth leader ahead of time' is the wordplay.
'youth leader' becomes 'y' (I've seen this in other clues).
'ahead of time' becomes 'early' (I've seen this before).
'y'+'early'='YEARLY'
(Other definitions for yearly that I've seen before include "Every 365 or 366 days" , "Approximately every 52 weeks" , "Every 12 months" , "in each revolution?" , "on a regular basis" .)