Cover round pool (4)
I believe the answer is:
'pool' is the definition.
(I've seen this before)
'cover round' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover**).
'round' becomes 'o' (round can mean a round object).
(Other definitions for lido that I've seen before include "Venice's beach island facing the Adriatic" , "Swim here" , "bathing facilities" , "Idol (anag.)" , "Public open swimming pool" .)