Diver circles a good lake (6)
I believe the answer is:
lagoon
'lake' is the definition.
(lagoon is a kind of lake)
'diver circles a good' is the wordplay.
'diver' becomes 'loon' (synonyms).
'circles' is an insertion indicator.
'good' becomes 'g' (abbreviation).
'a'+'g'='ag'
'loon' enclosing 'ag' is 'LAGOON'.
(Other definitions for lagoon that I've seen before include "Salt water lake enclosed by coral or sand" , "Calm water behind reef" , "Shallow salt-water lake" , "Almost enclosed body of water" , "Body of water cut off from a larger body by a reef" .)