Pure effluent reveals some rocks (4)
I believe the answer is:
reef
'rocks' is the definition.
(I've seen this before)
'pure effluent reveals some' is the wordplay.
'some' indicates a hidden word.
'REEF' is hidden in the letters of 'pure effluent reveals'.
(Other definitions for reef that I've seen before include "Submerged ridge of rock or coral near the surface" , "Rocks in shallow water - kind of knot" , "Coral-ridge" , "Rock near water's surface" , "Coral or rock just below sea surface" .)