Crossword Genius

Upsetting deficiency in measurement of energy provided (4)

Ross

I believe the answer is:

kcal

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

'measurement of energy provided' is the definition.
I can't tell whether this definition defines the answer.

'upsetting deficiency' is the wordplay.
'upsetting' is a reversal indicator.
'deficiency' becomes 'lack' (synonyms).
'lack' in reverse letter order is 'KCAL'.

'in' is the link.

Can you help me to learn more?

(Another definition for kcal that I've seen is " A measure of energy".)

I've seen this clue in The Irish Times.
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