Cover old swimming pool (4)
I believe the answer is:
lido
'swimming pool' is the definition.
(I've seen this before)
'cover old' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lid'+'o'='LIDO'
(Other definitions for lido that I've seen before include "Swimming bath" , "Swimming and leisure area" , "Idol (anag.)" , "Outdoor swimming pool" , "---- di Jesolo, Italian Adriatic resort" .)