Fine swimming pool covering's infallible
I believe the answer is:
'infallible' is the definition.
(similar in meaning)
'fine swimming pool covering's' is the wordplay.
'fine' becomes 'f' (abbreviation).
'swimming' is an anagram indicator.
'covering' becomes 'roof' (roof is a kind of covering).
'pool' is an anagram of 'oolp'.
(Other definitions for foolproof that I've seen before include "Infallible" , "incapable of going wrong" , "Certain" , "Not admitting failure" , "Can't be misinterpreted, even by simpleton" .)