Regatta's start and finish is Split (4)
I believe the answer is:
rend
'is split' is the definition.
(rend can mean to split or tear)
'regatta's start and finish' is the wordplay.
'start' indicates taking the first letters.
'and' means one lot of letters go next to another.
'finish' becomes 'end' (synonyms).
The initial letter of 'regattas' is 'r'.
'r'+'end'='REND'
(Other definitions for rend that I've seen before include "Tear apart, wrench" , "Nerd (anag.)" , "Tear violently" , "Tear to pieces" , "Tear apart roughly" .)