Needing exercise, one circles a furlong (5)
'needing exercise' is the definition.
(someone unfit might need to exercise)
'one circles a furlong' is the wordplay.
'one' becomes 'unit' (synonyms).
'circles' indicates putting letters inside.
'a furlong' becomes 'f' (abbreviation for furlong, a unit of length).
'unit' placed around 'f' is 'UNFIT'.
(Other definitions for unfit that I've seen before include "Inapt", "Not healthy", "Ill-suited", "Ill - out of shape", "Ill-equipped".)