Smashed eight runs - a certainty (1,4,5)

I believe the answer is:

a sure thing

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

'smashed eight runs' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs sometimes define each other.

'a certainty' is the wordplay.
'certainty' becomes 'sure thing' (I've seen this before).
'a'+'surething'='A SURE THING'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

Want a hint initially instead of a full solution? Install my app