A number of waves sound terrifying in boat (6)
I believe the answer is:
series
'a number' is the definition.
(thesaurus)
'terrifying in boat' is the wordplay.
'terrifying' becomes 'erie' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'boat' becomes 'ss'.
'erie' going inside 'ss' is 'SERIES'.
'of waves sound' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for series that I've seen before include "procession" , "Sets in sequence" , "Rows" , "Progression or sequence" , "Group of related radio/TV programmes" .)