Have the company of constables on a ship (7)
I believe the answer is:
possess
'have' is the definition.
(I know that have can be written as possess)
'company of constables on a ship' is the wordplay.
'company of constables' becomes 'posse' (I can't explain this - if you can you should believe this answer much more).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'a ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'posse'+'ss'='POSSESS'
'the' is the link.
Can you help me to learn more?
(Other definitions for possess that I've seen before include "Have and hold" , "influence" , "Have as one's own" , "Own, keep" , "Have ownership of" .)