Controlled line in river (4)
I believe the answer is:
Here is my best explanation:
'controlled' is the definition.
'line in river' is the subsidiary indication.
'line' becomes 'l'.
'in' means one lot of letters goes inside another.
'river' becomes 'cam' (river in Cambridge, England**).
'l' put into 'cam' is 'calm'.
(Other definitions for calm that I've seen before include "Absence of wind", "Peace, tranquillity", "At peace", "Halcyon", "Serene", "Still and quiet", "Still or peaceful", "Composure or serenity", "Cool, not excited" and "Composed and collected".)