Brief detailed description you gamble on? (4)

I believe the answer is:

spec

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

'gamble on?' is the definition.
'spec' can be an answer for 'gamble' (I've seen this before). I am not certain of the 'on?' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for spec that I've seen before include "Detailed design document (abbr.)" , "on which chance is taken" , "estimation?" , "On - means on the chance of achieving something" , "commercial venture" .)

I've seen this clue in The Times.

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