Sea snake one in container (7)
I believe the answer is:
caspian
'sea' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'snake one in container' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'container' becomes 'can' (can is a kind of container).
'asp'+'i'='aspi'
'aspi' put into 'can' is 'CASPIAN'.
Can you help me to learn more?
(Other definitions for caspian that I've seen before include "World's largest inland sea" , "-- Sea, inland lake" , "Prince --, Narnia book" .)