Runs in a race at the start, then has a breather (7)
I believe the answer is:
arrests
'runs in' is the definition.
(I've seen this in another clue)
'a race at the start then has a breather' is the wordplay.
'at the start' suggests taking the first letters.
'then' means one lot of letters go next to another.
'has a breather' becomes 'rests' (to have a breather is to take a rest).
The initial letter of 'race' is 'r'.
'a'+'r'+'rests'='ARRESTS'
(Other definitions for arrests that I've seen before include "Checks" , "collars" , "has a heart attack" , "made by police" , "Seizes and takes into custody" .)