Worst and best (4)
I believe the answer is:
beat
This clue is a double definition.
'worst' is the first definition.
(I've seen this before)
'best' is the second definition.
(I've seen this before)
'and' is the link.
(Other definitions for beat that I've seen before include "Musical rhythm" , "assigned patrol" , "club" , "Policeman's regular route" , "Police officer's allocated area to be patrolled" .)