Swimming pool disposing of old cover (3)
I believe the answer is:
lid
'cover' is the definition.
(I know that lid is a type of cover)
'swimming pool disposing of old' is the wordplay.
'swimming pool' becomes 'lido' (lido is a type of open-air swimming pool).
'disposing of' suggests deleting specific letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lido' with 'o' removed is 'LID'.
(Other definitions for lid that I've seen before include "Top for pot or jar" , "Top for bottle or eye protector" , "Top crust of pie" , "Bottle or jar top" , "Cap or top for container" .)