Completely occupy (4)
I believe the answer is:
fill
'completely occupy' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for fill that I've seen before include "Top up" , "Be suitable - . . . . the bill" , "Occupy the whole of" , "Repair tooth" , "` `Work expands to .... the time available' '" .)