Canon agonising to maintain figure (7)
I believe the answer is:
nonagon
'figure' is the definition.
(shape or figure with 9 sides)
'canon agonising to maintain' is the wordplay.
'to maintain' says the answer is hidden in the clue (the solution is kept or maintained by the clue).
'NONAGON' is hidden within 'canon agonising'.
(Other definitions for nonagon that I've seen before include "Nine-sided polygon" , "Nine sided figure" , "geometric figure" , "Nine-sided figure" , "Figure with nine, twenty-one Dn" .)