Son attempts to steal 500 Australian trousers (7)
I believe the answer is:
strides
'australian trousers' is the definition.
(Australian slang term for trousers)
'son attempts to steal 500' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'attempts' becomes 'tries' ('try' can be a synonym of 'attempt').
'to steal' is an insertion indicator.
'500' becomes 'D' (Roman numeral for 500).
'tries' placed around 'd' is 'trides'.
's'+'trides'='STRIDES'
(Other definitions for strides that I've seen before include "Walks fast" , "Walks, trousers (colloq.)" , "Takes great steps" , "Daks" , "footsteps" .)