Cover old pool (4)
I believe the answer is:
lido
'pool' is the definition.
(I know that Lido is a type of lido)
'cover old' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lid'+'o'='LIDO'
(Other definitions for lido that I've seen before include "---- di Jesolo, Italian Adriatic resort" , "Open-air swimming-pool" , "Aquatic leisure centre" , "Open-air swimming pool" , "Venetian resort" .)