Plimsoll (4)
'plimsoll' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sandshoe that I've seen before include "Plimsoll" , "footwear" .)
(Other definitions for pump that I've seen before include "Obtain information from - a shoe" , "Raise liquid or air" , "Set foot in it" , "Device for moving fluids" , "Fluid mover" .)