Diver circling silver lake (6)
I believe the answer is:
lagoon
'lake' is the definition.
(lagoon is a kind of lake)
'diver circling silver' is the wordplay.
'diver' becomes 'loon' ('loon' can be a synonym of 'diver').
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'silver' becomes 'ag' (Ag is the chemical symbol for silver).
'loon' enclosing 'ag' is 'LAGOON'.
(Other definitions for lagoon that I've seen before include "Area inside atoll" , "Shallow salt-water lake" , "... separating off this water?" , "Lake inside coral reef" , "Calm water behind reef" .)