Watertight chamber in America is so narrow (7)
I believe the answer is:
caisson
'watertight chamber' is the definition.
(I've seen this before)
'in america is so narrow' is the wordplay.
'in' says the answer is hidden in the clue.
'CAISSON' can be found hidden inside 'america is so narrow'.
(Other definitions for caisson that I've seen before include "Water-excluding case" , "So I scan the ammunition wagon" , "Structure excluding water from construction" , "watertight chamber" .)