Narrow water, the line to take in crossing (5)
I believe the answer is:
fiord
'narrow water the line' is the definition.
I know that 'narrow' relates to this answer.
'take in crossing' is the wordplay.
'take' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'crossing' becomes 'ford' (ford is a kind of crossing).
'i' put inside 'ford' is 'FIORD'.
'to' is the link.
Can you help me to learn more?
(Other definitions for fiord that I've seen before include "Long narrow sea inlet between high cliffs" , "Long, steep inlet from the sea" , "Deep sea inlet as in Norway" .)