Crucial moment of stomach workout? (6,4)

I believe the answer is:

crunch time

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'stomach workout?' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'crucial moment' is the wordplay.
'crucial' becomes 'crunch' (I am not sure about this - if you are sure you should believe this answer much more).
'moment' becomes 'time' (time is a kind of moment).
'crunch'+'time'='CRUNCH TIME'

'of' is the link.

Can you help me to learn more?

I've seen this clue in The Times.

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