It's after a slow start that one takes a rest (4)
I believe the answer is:
sits
'takes a rest' is the definition.
The answer and definition can be both to do with social activities as well as being verbs in their -s form.
Perhaps you can see an association between them that I can't see?
'it's after a slow start that one' is the wordplay.
'it's' becomes 'ts' (abbreviation. e.g. in 'tis).
'after' is a charade indicator (letters next to each other).
'start' indicates taking the first letters.
'that one' becomes 'I' (Roman numeral).
The first letter of 'slow' is 's'.
's'+'i'='si'
'ts' after 'si' is 'SITS'.
Can you help me to learn more?
(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" .)