They would be crowned before getting on boats (10)

I believe the answer is:

pretenders

'they would be crowned' is the definition.
(people who want to be crowned monarch)

'before getting on boats' is the wordplay.
'before' becomes 'pre' (prefix meaning 'before').
'getting on' says to put letters next to each other (in a down clue, some letters go on top of others).
'boats' becomes 'tenders' (tender is a kind of boat).
'pre'+'tenders'='PRETENDERS'

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.