Establish ascendancy over naval vessel, expected to arrive (6)
I believe the answer is:
subdue
'establish ascendancy' is the definition.
I can't judge whether this definition defines the answer.
'over naval vessel expected to arrive' is the wordplay.
'over' means one lot of letters go next to another.
'naval vessel' becomes 'sub'.
'expected to arrive' becomes 'due' (I've seen this in another clue).
'sub'+'due' is 'SUBDUE'.
Can you help me to learn more?
(Other definitions for subdue that I've seen before include "'Quieten, bring under control (6)'" , "Overcome, bring under control" , "Force to yield, tame" , "Moderate" , "Conquer" .)