Amphibious commando in ocean and lake (4)
I believe the answer is:
seal
'amphibious commando in' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'ocean and lake' is the wordplay.
'ocean' becomes 'sea' (synonyms).
'and' is a charade indicator (letters next to each other).
'lake' becomes 'L' (geographical abbreviation).
'sea'+'l'='SEAL'
Can you help me to learn more?
(Other definitions for seal that I've seen before include "Mammal; impressed wax" , "marine creature" , "Marine animal or sign of authenticity" , "Imprinted disk affixed to a document" , "See 7" .)