Mathematical symbol displayed on a blackboard (5)

I believe the answer is:

nabla

'mathematical symbol' is the definition.

'displayed on a blackboard' is the wordplay.
'displayed' says the answer is hidden in the clue (the answer is displayed within the clue).
'NABLA' can be found hidden inside 'on a blackboard'.

(Another definition for nabla that I've seen is " Del".)

I've seen this clue in The Guardian.

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