Unruffled learner entering river (4)
I believe the answer is:
calm
'unruffled' is the definition.
('calm' can be similar in meaning to 'unruffled')
'learner entering river' is the wordplay.
'learner' becomes 'l'.
'entering' indicates putting letters inside.
'river' becomes 'cam' (I've seen this before**).
'l' inserted into 'cam' is 'CALM'.
(Other definitions for calm that I've seen before include "Tranquil, undisturbed" , "Composed or free from storms" , "Still and quiet" , "Absence of wind" , "Self-possessed" .)