Seize gang on ship (7)
I believe the answer is:
possess
'seize' is the definition.
(thesaurus)
'gang on ship' is the wordplay.
'gang' becomes 'posse' (I've seen this in another clue).
'on' is a charade indicator (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 "Have - control" , "Own, keep" , "Have as one's own" , "Have and hold" , "Have ownership of" .)