Swimming race Thai regularly enters (8)
I believe the answer is:
natation
'swimming' is the definition.
(I know that natation is a type of swimming)
'race thai regularly enters' is the wordplay.
'race' becomes 'nation' (both can mean a national or ethnic group).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'enters' means one lot of letters goes inside another.
The alternate letters of 'thai' are 'ta'.
'nation' placed around 'ta' is 'NATATION'.
(Other definitions for natation that I've seen before include "Swimming" , "swimmingly" .)