One searching smallest room, say, for complaint in the Observer (7)

I believe the answer is:

leucoma

'complaint in the observer' is the definition.
(problem with the eye, something which observes)

I don't understand how the remainder of the clue works.

(Other definitions for leucoma that I've seen before include "Eye condition" , "Opacity of the cornea - Mao clue (anag)" .)

I've seen this clue in The Guardian.

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