Narrow sea inlet (5)
'narrow sea inlet' is the definition.
'fjord' can be an answer for 'inlet' (fjord is a kind of inlet). I also know that both 'narrow' and 'sea' relate to this answer.
This is all the clue.
(Other definitions for fjord that I've seen before include "Long narrow inlet" , "Norwegian inlet (J, not I)" , "Sea inlet" , "Northern location?" , "Rock-bound inlet" .)
'narrow sea inlet' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for firth that I've seen before include "Estuary" , "Narrow sea inlet in Scotland" , "Scottish sea-inlet" , "Scottish river mouth" , "Arm of the sea - actor Colin" .)