Don’t start to hurt small parts of body (4)
I believe the answer is:
arms
'parts of body' is the definition.
'don't start to hurt small' is the wordplay.
'don't start' means to remove the first letter.
'to hurt' becomes 'harm' (synonyms).
'small' becomes 's' (abbreviation - e.g. clothes size).
'harm' with its initial letter taken off is 'arm'.
'arm'+'s'='ARMS'
(Other definitions for arms that I've seen before include "Guns et cetera" , "See 12" , "Weapons, collectively" , "Ordnance" , "International competition for weapons" .)