Calm seen off with hesitation (6)
I believe the answer is:
serene
'calm' is the definition.
('serene' can be a synonym of 'calm')
'seen off with hesitation' is the wordplay.
'off' indicates an anagram.
'with' indicates putting letters inside.
'hesitation' becomes 'er'.
'seen' anagrammed gives 'sene'.
'sene' placed around 'er' is 'SERENE'.
(Other definitions for serene that I've seen before include "Tranquil, perfectly calm" , "Calm and unruffled, like a princely title." , "without any trouble" , "Not agitated" , "Peaceful and unruffled" .)