Irritating and spiteful, but not at first (5)
I believe the answer is:
'irritating' is the definition.
(an itch is irritating)
'spiteful but not at first' is the wordplay.
'spiteful' becomes 'bitchy' (informal term).
'but not at first' means to remove the first letter.
'bitchy' with its first letter removed is 'ITCHY'.
'and' acts as a link.
(Other definitions for itchy that I've seen before include "Feeling as if one wants to scratch" , "Tingling - desirous" , "Irritable" , "Affected by desire" , "Unable to relax" .)