Round some work up, nasty sort (4)
I believe the answer is:
ogre
'nasty sort' is the definition.
(ogre can mean a nasty person)
'round some work up' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'some work' becomes 'erg' (an erg is a unit of work done).
'up' is a reversal indicator (in down clue: letters go upwards).
'erg' in reverse letter order is 'gre'.
'o'+'gre'='OGRE'
(Other definitions for ogre that I've seen before include "Human-eating giant" , "Ergo (anag.)" , "man-eater" , "Terrifying person" , "Wicked giant" .)