Part of the arena was hastily covered with water (5)
I believe the answer is:
awash
'covered with water' is the definition.
(I've seen this before)
'part of the arena was hastily' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'AWASH' is hidden within 'arena was hastily'.
(Other definitions for awash that I've seen before include "Overflowing" , "With the top just covered with water" , "Level with the water surface" , "Doused in water" , "Letting the 21 in" .)