Essential oil in the Parramatta River? (5)
I believe the answer is:
attar
'essential oil' is the definition.
(attar is a kind of essential oil)
'in the parramatta river?' is the wordplay.
'in' indicates a hidden word.
'ATTAR' can be found hidden inside 'parramatta river'.
(Other definitions for attar that I've seen before include "Fragrant essence usually associated with the damask rose" , "Send out roses" , "Oil from flowers for perfume" , "Fragrant essential oil from damask roses" , "Fragrant rose oil" .)