Fixation with school reunion? (9)

I believe the answer is:

obsession

'fixation' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for obsession that I've seen before include "Fixed idea" , "Phobia" , "All-absorbing idea" , "Continual preoccupation" , "Mania" .)

I've seen this clue in The Independent.

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