Nasty piece of work that is below average (6)
I believe the answer is:
meanie
'nasty piece of work' is the definition.
'that is below average' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'below' is a charade indicator (letters next to each other).
'average' becomes 'mean' (type of average).
'ie' after 'mean' is 'MEANIE'.
(Other definitions for meanie that I've seen before include "Small-minded type" , "Ungenerous person" , "Unkind person (informal)" , "Nasty piece of work?" , "Selfish type (informal)" .)