Cover over swimming pool (4)
I believe the answer is:
lido
'swimming pool' is the definition.
(lido is a type of open-air swimming pool)
'cover over' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover).
'over' becomes 'o' (cricket abbreviation).
'lid'+'o'='LIDO'
(Other definitions for lido that I've seen before include "Swimming and leisure area" , "---- di Jesolo, Italian Adriatic resort" , "bathing-beach" , "Open-air swimming-pool" , "Italian bathing place" .)