Gandhi isn't part of hidden agenda that's dull (6)

I believe the answer is:

deaden

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'that's dull' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for deaden that I've seen before include "Check" , "Damp down; make less sensitive" , "Blunt, numb" , "Dampen" , "Take life" .)

I've seen this clue in The Irish Times.

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