Still large number in the river (4)
I believe the answer is:
calm
'still' is the definition.
('calm' can be a synonym of 'still')
'large number in the river' is the wordplay.
'large number' becomes 'l'.
'in the' means one lot of letters goes inside another.
'river' becomes 'cam' (I've seen this before**).
'l' placed inside 'cam' is 'CALM'.
(Other definitions for calm that I've seen before include "Composed and collected" , "Self-possessed" , "Peaceful and unexcited" , "Serene" , "stained-glass window maker could need this" .)