Grounds sailors, say, in rising currents (8)
I believe the answer is:
sediment
'grounds' is the definition.
(thesaurus)
'sailors say in rising currents' is the wordplay.
'sailors say' becomes 'men' (sailor is a kind of man).
'in' indicates putting letters inside.
'rising' is a reversal indicator (in a down clue, letters go up).
'currents' becomes 'tides' (I've seen this in another clue).
'tides' written backwards gives 'sedit'.
'men' placed within 'sedit' is 'SEDIMENT'.
(Other definitions for sediment that I've seen before include "Particles that settle on the bottom of river or wine" , "Matter settling out in fluids" , "Stuff that settles to the bottom" , "Matter settling out in liquid" , "End times (anag)" .)