American commando in ocean and lake (4)
I believe the answer is:
seal
'american commando' is the definition.
(US Navy SEALs)
'ocean and lake' is the wordplay.
'ocean' becomes 'sea' (synonyms).
'and' says to put letters next to each other.
'lake' becomes 'L' (geographical abbreviation).
'sea'+'l'='SEAL'
'in' is the link.
(Other definitions for seal that I've seen before include "Stamp of approval" , "Member of an elite force within the US Navy" , "Make tight" , "Ocean mammal or official stamp." , "Stamped wax; mammal" .)