Calm observed round River Ribble finally (6)
I believe the answer is:
serene
'calm' is the definition.
(synonyms)
'observed round river ribble finally' is the wordplay.
'observed' becomes 'seen' (synonyms).
'round' means one lot of letters goes inside another.
'river' becomes 'r'.
'finally' indicates one should take the final letters.
The last letter of 'ribble' is 'e'.
'seen' going around 'r' is 'seren'.
'seren'+'e'='SERENE'
(Other definitions for serene that I've seen before include "Placid" , "Calm and at peace" , "Impassive" , "Calm and unruffled, like a princely title." , "Peaceful and unruffled" .)