River’s course under the sea (6)
I believe the answer is:
medway
'river's' is the definition.
(river of south-east England)
'course under the sea' is the wordplay.
'course' becomes 'way' (way is a kind of course**).
'under' is a charade indicator (letters next to each other).
'the sea' becomes 'med' (abbreviation of Mediterranean).
'way' after 'med' is 'MEDWAY'.
(Other definitions for medway that I've seen before include "River at Maidstone" , "River of Kent" .)