Marlin's milieu (3)
I believe the answer is:
sea
'marlin's milieu' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sea that I've seen before include "The Aegean, for example" , "Caspian, e.g." , "7 [ship] s on this" , "Ocean area" , "Division of an ocean" .)