Ruin a runaway affair (7)

I believe the answer is:

scupper

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'ruin a runaway affair' is the definition.
'scupper' can be an answer for 'ruin' (I've seen this before). I'm not certain of the 'a runaway affair' bit.

'ruin a' is the wordplay.
'ruin' becomes 'scup' (I can't explain this - if you can you should believe this answer much more).
'a' becomes 'per' (eg 'one a day' means 'one per day').
'scup'+'per'='SCUPPER'

Can you help me to learn more?

(Other definitions for scupper that I've seen before include "Ruin (plans)" , "Ruin, put paid to" , "hole in its side" , "Deliberately sink (oneself)" , "dish" .)

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

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