Controlled line in river (4)
I believe the answer is:
calm
'controlled' is the definition.
(I've seen this before)
'line in river' is the wordplay.
'line' becomes 'l'.
'in' is an insertion indicator.
'river' becomes 'cam' (I've seen this before**).
'l' placed within 'cam' is 'CALM'.
(Other definitions for calm that I've seen before include "Tranquil, undisturbed" , "Controlled, not agitated" , "Pacific" , "Peaceful, not excited" , "Peaceful, undisturbed" .)