Receding flow of water over stone (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'receding flow of water over' is the wordplay.
'receding' says the letters should be written in reverse.
'flow of water' becomes 'lap' (as in lapping water).
'over' becomes 'o' (cricket abbreviation).
'lap'+'o'='lapo'
'lapo' in reverse letter order is 'OPAL'.
(Other definitions for opal that I've seen before include "Milk-coloured gemstone" , "Quartz-like gemstone" , "Translucent gemstone" , "Opaque gem" , "Milky-white gemstone with flashes of colour" .)