Substantial complaint by youth leader (5)
I believe the answer is:
beefy
'substantial' is the definition.
(I've seen this in another clue)
'complaint by youth leader' is the wordplay.
'complaint' becomes 'beef' (beef can mean a complaint or objection).
'by' says to put letters next to each other.
'youth leader' becomes 'y' (I've seen this in other clues).
'beef'+'y'='BEEFY'
(Other definitions for beefy that I've seen before include "Brawny" , "Solid" , "Athletic and muscular (non-vegetarian?)" , "like some bully?" , "Muscular, robust" .)