Bag first of berths in ship (3,5)

I believe the answer is:

bin liner

'bag' is the definition.
(bag for a rubbish bin)

'first of berths in ship' is the wordplay.
'first of berths' becomes 'b' (1st letter of 'berths').
'ship' becomes 'liner' (liner is a kind of ship).
'b'+'in'+'liner'='BIN-LINER'

(Another definition for bin liner that I've seen is " Rubbish holder inside a rubbish holder".)

I've seen this clue in The Guardian.

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