He’s small-minded, average, extremely irksome (6)
I believe the answer is:
meanie
'he's small-minded' is the definition.
(a meanie is a small-minded or unkind person)
'average extremely irksome' is the wordplay.
'average' becomes 'mean' (type of average).
'extremely' means to remove the middle letters.
'irksome' with its middle taken out is 'ie'.
'mean'+'ie'='MEANIE'
(Other definitions for meanie that I've seen before include "Selfish type (informal)" , "Stingy person (informal)" , "nasty person" , "Ungenerous person" , "Unkind person (informal)" .)