The chest of a big lad? (7)
I believe the answer is:
tallboy
'the chest' is the definition.
(tallboy is a kind of chest)
'big lad?' is the wordplay.
'big' becomes 'tall' ().
'lad?' becomes 'boy' (lad is a kind of boy).
'tall'+'boy'='TALLBOY'
'of a' acts as a link.
(Other definitions for tallboy that I've seen before include "Type of chest of drawers" , "Chest of drawers in two sections" , "Two-part chest of drawers" , "Cabinet" , "Chest of drawers for lofty lad" .)