Not at first spiteful, but irritating (5)
I believe the answer is:
itchy
'irritating' is the definition.
(an itch is irritating)
'not at first spiteful' is the wordplay.
'not at first' means to remove the first letter.
'spiteful' becomes 'bitchy' (I've seen this before).
'bitchy' with its first letter removed is 'ITCHY'.
'but' is the link.
(Other definitions for itchy that I've seen before include "Irritating to the skin" , "Wanting a scratch?" , "Like the feet of the restless" , "Bothersome" , "Causing irritation to skin" .)