Too low a calculation for me, trade unionist sort-of (15)

I believe the answer is:

underestimation

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'too low a calculation' is the definition.
I can't tell whether this defines the answer.

'me trade unionist sort-of' is the wordplay.
'sort of' indicates an anagram.
'me'+'trade'+'unionist'='metradeunionist'
'metradeunionist' anagrammed gives 'UNDERESTIMATION'.

'for' acts as a link.

Can you help me to learn more?

(Another definition for underestimation that I've seen is " deficient appraisal".)

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

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