Filthy tables arranged by youth leader (7)
I believe the answer is:
beastly
'filthy' is the definition.
(synonyms)
'tables arranged by youth leader' is the wordplay.
'arranged' indicates an anagram.
'by' means one lot of letters go next to another.
'youth leader' becomes 'y' (I've seen this in other clues).
'tables' anagrammed gives 'beastl'.
'beastl'+'y'='BEASTLY'
(Other definitions for beastly that I've seen before include "Very unpleasant" , "Horrid" , "Bestial" , "Brutal - foul" , "Brutish" .)