Top pool that's bottomless (3)
I believe the answer is:
lid
'top' is the definition.
(I know that lid is a type of top)
'pool that's bottomless' is the wordplay.
'pool' becomes 'lido' (I've seen this before).
'that's bottomless' means to remove the last letter.
'lido' with its final letter taken away is 'LID'.
(Other definitions for lid that I've seen before include "Covering" , "Pie crust; cover" , "Top crust of pie" , "Cover from gliders" , "Top for bottle or eye protector" .)