One trap set around river still (5)
I believe the answer is:
inert
'still' is the definition.
(not moving)
'one trap set around river' is the wordplay.
'one' becomes 'i' (Roman numeral).
'trap' becomes 'net' (net is a kind of trap).
'set around' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'net' placed around 'r' is 'nert'.
'i'+'nert'='INERT'
(Other definitions for inert that I've seen before include "lacking energy" , "Nothing doing" , "Unable to move or resist motion" , "Inter, being unable to move" , "Slow and apathetic" .)