About to probe Australia, turning up nothing (4)
I believe the answer is:
zero
'nothing' is the definition.
(I know that nothing can be written as zero)
'about to probe australia turning up' is the wordplay.
'about' becomes 're' (regarding).
'to probe' means one lot of letters goes inside another (probe can mean to poke or prod into).
'australia' becomes 'oz' (informal name for Australia).
'turning up' shows that the letters should be reversed in order.
're' put inside 'oz' is 'orez'.
'orez' in reverse letter order is 'ZERO'.
(Other definitions for zero that I've seen before include "Nowt" , "Love" , "Worthless person" , "You'll find nothing" , "Naught" .)