Crossword Genius

A runaway affair on ship? (7)

Ross

I believe the answer is:

scupper

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'ship?' is the definition.
(thesaurus)

'a runaway affair on' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'runaway affair' becomes 'scup' (I can't explain this - if you can you should believe this answer much more).
'on' means one lot of letters go next to another.
'per' put after 'scup' is 'SCUPPER'.

Can you help me to learn more?

(Other definitions for scupper that I've seen before include "Cause to sink" , "foil" , "dish" , "Deliberately sink (oneself)" , "Ruin (plans)" .)

I've seen this clue in The Globe and Mail.
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