Between seas - that sounds direct (6)
I believe the answer is:
strait
'between seas' is the definition.
I can't judge whether this defines the answer.
'sounds direct' is the wordplay.
'sounds' indicates a 'sounds like' (homophone) clue.
'direct' becomes 'straight' (synonyms).
'straight' is a homophone of 'STRAIT'.
'that' is the link.
Can you help me to learn more?
(Other definitions for strait that I've seen before include "Sound" , "Narrow sea passage" , "watery scene?" , "Difficult position" , "Narrow kind of traits" .)