Man races in circles (4)
I believe the answer is:
otto
'man' is the definition.
(I have seen 'A man' mean 'otto' so perhaps 'man' could also mean 'otto')
'races in circles' is the wordplay.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'in' means one lot of letters goes inside another.
'circles' becomes 'oo' (resembles two O's).
'tt' placed within 'oo' is 'OTTO'.
(Other definitions for otto that I've seen before include "he goes" , "perfume ingredient" , "____ Preminger, US film director born in Austria" , "- Preminger, Austrian-born US film producer" , "Name of four German kings" .)