It's reasonable, if only one can see it (5)

I believe the answer is:

sense

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'it's reasonable' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for sense that I've seen before include "Hearing maybe" , "See 16" , "Meaning - feeling" , "Become aware of" , "Feeling - reason" .)

I've seen this clue in the Evening Standard.

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