Swimming pool cover is old (4)
I believe the answer is:
lido
'swimming pool' is the definition.
(lido is a type of open-air swimming pool)
'cover is old' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover).
'is 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" , "Aquatic leisure centre" , "Public bathing beach" , "Swimming and leisure area" , "Public open swimming pool" .)