Notice I alter bad swimming pool equipment (8,5)
I believe the answer is:
billiard table
'pool equipment' is the definition.
(equipment for a game of pool)
'notice i alter bad swimming' is the wordplay.
'notice' becomes 'bill' ('bill' can be a synonym of 'notice'**).
'swimming' indicates an anagram.
'alter'+'bad'='alterbad'
'alterbad' anagrammed gives 'ardtable'.
'bill'+'i'+'ardtable'='BILLIARD-TABLE'
(Other definitions for billiard table that I've seen before include "Flat baize-covered surface with pockets" , "Six-pocketed games surface" , "Large flat item used for indoor game" , "What has pockets and cushions" , "Surface for pool, snooker, etc" .)