Spiteful leader of band — irritating! (6)
I believe the answer is:
bitchy
'spiteful' is the definition.
(informal term)
'leader of band irritating' is the wordplay.
'leader of' indicates taking the first letters.
'irritating' becomes 'itchy' (an itch is irritating).
The first letter of 'band' is 'b'.
'b'+'itchy'='BITCHY'
(Other definitions for bitchy that I've seen before include "Speaking spitefully in manner of female canine" , "Catty" , "Unpleasant and spiteful like female dog" , "malicious" .)