Agree with a lofty youth leader (5)
I believe the answer is:
tally
'agree' is the definition.
(synonyms)
'with a lofty youth leader' is the wordplay.
'with' is a charade indicator (letters next to each other).
'a lofty' becomes 'tall' (I've seen this before).
'youth leader' becomes 'y' (I've seen this in other clues).
'tall'+'y' is 'TALLY'.
(Other definitions for tally that I've seen before include "highly?" , "Square" , "Mark" , "Agree - count" , "Coincide" .)