Something requiring a snap decision

I believe the answer is:

photo finish

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

'something requiring a' is the definition.
I can't judge whether this definition defines the answer.

'snap decision' is the wordplay.
'snap' becomes 'photo' (I've seen this before).
'decision' becomes 'finish' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'photo'+'finish'='PHOTO FINISH'

Can you help me to learn more?

(Other definitions for photo finish that I've seen before include "Virtual dead heat in a horse race" , "Close ending to a race" .)

I've seen this clue in The Telegraph.

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