It's silly to be spiteful to a slow starter (6)
I believe the answer is:
scatty
'it's silly' is the definition.
(scatty means silly or absent-minded)
'spiteful to a slow starter' is the wordplay.
'spiteful' becomes 'catty' (catty can mean spiteful or nasty).
'to' says to put letters next to each other (I've seen this in other clues).
'starter' says to take the initial letters.
The first letter of 'slow' is 's'.
'catty' put after 's' is 'SCATTY'.
'to be' is the link.
(Other definitions for scatty that I've seen before include "Slightly crazy" , "Not thinking straight" , "Informally crazy or feather-headed" , "Informally silly or crazy" , "Feather-brained" .)