Explanatory figure lacking a couple of letters together (6)

I believe the answer is:

digram

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'couple of letters together' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'explanatory figure lacking a' is the wordplay.
'explanatory figure' becomes 'diagram' (I've seen this before).
'lacking' is a deletion indicator.
'diagram' with 'a' taken out is 'DIGRAM'.

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I've seen this clue in The Guardian.

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