Cover over pool (4)
I believe the answer is:
lido
'pool' is the definition.
(I know that lido is a type of outdoor 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 "---- di Jesolo, Italian Adriatic resort" , "Public bathing beach" , "Venetian resort" , "Public open swimming pool" , "bathing-beach" .)