Fools go backwards in health resorts (4)
I believe the answer is:
spas
'health resorts' is the definition.
(I've seen this in another clue)
'fools go backwards' is the wordplay.
'fools' becomes 'saps' ('sap' can be a synonym of 'fool').
'go backwards' is a reversal indicator (I've seen 'going backwards' mean this).
'saps' back-to-front is 'SPAS'.
'in' acts as a link.
(Other definitions for spas that I've seen before include "Mineral springs" , "Bath and suchlike" , "Health resorts near springs" .)