Singular athlete, one accustomed to beam (6)
I believe the answer is:
smiler
'one accustomed to beam' is the definition.
(someone who smiles or beams)
'singular athlete' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'athlete' becomes 'miler' (person trained to run a mile).
's'+'miler'='SMILER'
(Other definitions for smiler that I've seen before include "Cheerful-looking character" , "Happy chappie" , "one looks quite happy" .)