Occasional Australian turning up in US state (6)
I believe the answer is:
casual
'occasional' is the definition.
(I know that occasional can be written as casual)
'australian turning up in us state' is the wordplay.
'australian' becomes 'aus' (common abbreviation).
'turning up' is a reversal indicator.
'in' means one lot of letters goes inside another.
'us state' becomes 'Cal' (abbreviation for California).
'aus' in reverse letter order is 'sua'.
'sua' placed inside 'cal' is 'CASUAL'.
(Other definitions for casual that I've seen before include "Without formality" , "Informal or not showing commitment" , "Relaxed - unintentional" , "Informal or not showing any enthusiasm" , "Informal or cursory" .)