Sailor’s secure? He might get the sack (6)
I believe the answer is:
'he might get the sack' is the definition.
(a postman carries a sack of letters)
'sailor's secure?' is the wordplay.
'sailor's' becomes 'PO's' (petty officer).
'secure?' becomes 'tie' (tying is a kind of securing).
(Another definition for postie that I've seen is " Mail deliverer (informal)".)