Seize a body of constables on ship (7)
I believe the answer is:
possess
'seize' is the definition.
(thesaurus)
'a body of constables on ship' is the wordplay.
'a body of constables' becomes 'posse' (I've seen this before).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'posse'+'ss'='POSSESS'
(Other definitions for possess that I've seen before include "Consume" , "influence" , "Have ownership of" , "Have and hold" , "Own, keep" .)