Ultimately reject bad work (4)
I believe the answer is:
till
'work' is the definition.
(to till is to work the land)
'ultimately reject bad' is the wordplay.
'ultimately' says to take the final letters.
'bad' becomes 'ill' (similar in meaning).
The last letter of 'reject' is 't'.
't'+'ill'='TILL'
(Other definitions for till that I've seen before include "Small box in shop for holding cash" , "Cultivate - cash drawer" , "there's money in it" , "Money box" , "Prepare land for cultivation by ploughing etc" .)