Lad goes round Australia having plenty of drinks (5)
I believe the answer is:
boozy
'having plenty of drinks' is the definition.
(having lots of alcohol)
'lad goes round australia' is the wordplay.
'lad' becomes 'boy' (lad means a young boy).
'goes round' means one lot of letters goes inside another.
'australia' becomes 'oz' (informal name for Australia).
'boy' placed around 'oz' is 'BOOZY'.
(Other definitions for boozy that I've seen before include "Drinking lots of alcohol" , "Happy" , "Alcoholic" , "Dionysian" .)