Swimming race — a time to dive in (8)
I believe the answer is:
natation
'swimming' is the definition.
(I know that natation is a type of swimming)
'race a time to dive in' is the wordplay.
'race' becomes 'nation' (both can mean a national or ethnic group).
'time' becomes 't' (abbreviation).
'to dive in' is an insertion indicator.
'a'+'t'='at'
'nation' going around 'at' is 'NATATION'.
(Other definitions for natation that I've seen before include "Swimming" , "swimmingly" .)