Cover over very large leisure facilities (5)
I believe the answer is:
lidos
'leisure facilities' is the definition.
(I've seen this before)
'cover over very large' is the wordplay.
'cover' becomes 'lid' (lid is a kind of cover).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'very large' becomes 'os' (abbreviation for oversize).
'lid'+'os'='LIDOS'
(Other definitions for lidos that I've seen before include "bathing beaches" , "which swimmers may use" , "Open air swimming pools" , "swimmers like them" , "Solid (anag) -- pools" .)