Being calm, sneer wryly as the race finishes (6)
I believe the answer is:
serene
'being calm' is the definition.
('serene' can be a synonym of 'calm')
'sneer wryly as the race finishes' is the wordplay.
I cannot quite see how this works, but
an anagram of 'sneer' is 'seren' which is located in the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for serene that I've seen before include "Calm and at peace" , "Calm and unruffled, like a princely title." , "Fair" , "Not agitated" , "still" .)