Times keeping us hard at work (4)
I believe the answer is:
busy
'hard at work' is the definition.
'times keeping us' is the wordplay.
'times' becomes 'by' (I've seen this in other clues).
'keeping' indicates putting letters inside.
'by' going around 'us' is 'BUSY'.
(Other definitions for busy that I've seen before include "crowded" , "Fully occupied" , "With much to do" , "With lots to do" , "detective" .)