Missiles located below northern straits (7)
I believe the answer is:
narrows
'straits' is the definition.
(I know this)
'missiles located below northern' is the wordplay.
'missiles' becomes 'arrows' (arrow is a kind of missile).
'located below' says to put letters next to each other.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'arrows' after 'n' is 'NARROWS'.
(Other definitions for narrows that I've seen before include "Confined channel" , "Sound" , "Gets smaller in width" , "Loses width" , "Thin straits connecting two bodies of water" .)