Passing Sian frolicking in river (9)
I believe the answer is:
transient
'passing' is the definition.
('transient' can be a synonym of 'passing')
'sian frolicking in river' is the wordplay.
'frolicking' is an anagram indicator.
'in' means one lot of letters goes inside another.
'river' becomes 'trent' (I've seen this before**).
'sian' is an anagram of 'ansi'.
'ansi' placed within 'trent' is 'TRANSIENT'.
(Other definitions for transient that I've seen before include "gone in a trice!" , "Temporary worker" , "Lasting little time" , "fugitive" , "Ephemeral" .)