Call off when the boat capsizes about the start of the race (5)
I believe the answer is:
abort
'call off' is the definition.
(I've seen this in another clue)
'the boat capsizes about the start of the race' is the wordplay.
'capsizes' indicates anagramming the letters (I've seen 'capsizing' mean this).
'about' means one lot of letters goes inside another.
'the start of' indicates taking the first letters.
The first letter of 'race' is 'r'.
'boat' is an anagram of 'abot'.
'abot' placed around 'r' is 'ABORT'.
'when' acts as a link.
(Other definitions for abort that I've seen before include "End prematurely" , "Cut short" , "Terminate prematurely" , "Bring to premature conclusion" , "Pull the plug on" .)