Extra aboard ship (Lusitania) (4)

I believe the answer is:

plus

'extra' is the definition.
(plus can mean more than expected)

'aboard ship lusitania' is the wordplay.
'aboard' says the answer is hidden in the clue (the answer is aboard or within the clue).
'PLUS' can be found hidden inside 'ship lusitania'.

(Other definitions for plus that I've seen before include "Add" , "being a little cross" , "Something more" , "Furthermore" , "And, arithmetically" .)

I've seen this clue in The Telegraph.

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