Lounge has to include youth (6)
I believe the answer is:
laddie
'youth' is the definition.
(thesaurus)
'lounge has to include' is the wordplay.
'lounge' becomes 'lie' (I've seen this in other clues).
'has' means one lot of letters goes inside another.
'to include' becomes 'add' (I've seen this before).
'lie' placed around 'add' is 'LADDIE'.
(Other definitions for laddie that I've seen before include "Child" , "Youngster" , "Scottish boy" , "Sonny" , "(Scots) boy" .)