Extremely harmful ballgame in America? Nothing in it (7)

I believe the answer is:

ruinous

I'm a little stuck... Click here to teach me more about this clue!

'extremely harmful' is the definition.
'ruinous' can be an answer for 'harmful' (similar in meaning). I am not sure about the 'extremely' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for ruinous that I've seen before include "Devastating - dilapidated" , "Catastrophic" , "causing harm?" , "Disastrous, destructive" , "Destructive and causing downfall" .)

I've seen this clue in The Sunday Times and The Times.

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