Some tarpon deep in artificial lake (4)
I believe the answer is:
pond
'artificial lake' is the definition.
(thesaurus)
'some tarpon deep in' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'POND' is hidden within 'tarpon deep in'.
(Other definitions for pond that I've seen before include "Small area of still water" , "Pool of still water" , "Possible site for weed" , "Small lake" , "Himalayan country" .)