Maybe one-to-one operation is somewhat lacking (5)

I believe the answer is:

ratio

'maybe one-to-one' is the definition.
(1 to 1 is a ratio)

'operation is somewhat lacking' is the wordplay.
'is somewhat lacking' indicates the answer is hidden within the clue (some letters removed to reveal the answer).
'RATIO' can be found hidden inside 'operation'.

(Other definitions for ratio that I've seen before include "3 relation" , "Proportional relation between two numbers" , "figures in relationship" , "Relative proportion" , "Comparative measure" .)

I've seen this clue in The Times.

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