It's after a slow start that one takes a rest (4)
I believe the answer is:
sits
'one takes a rest' is the definition.
(I've seen this in another clue)
'it's after a slow start' is the wordplay.
'it's' becomes 'its' ().
'after' is a charade indicator (letters next to each other).
'start' suggests taking the first letters.
The first letter of 'slow' is 's'.
'its' after 's' is 'SITS'.
'that' acts as a link.
(Other definitions for sits that I've seen before include "Takes a chair" , "Perches" , "Takes an exam" , "Poses for portrait" , "Takes a seat for the painter" .)