Is it reasonable to get off frigate? (4)

I believe the answer is:

fair

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'frigate?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for fair that I've seen before include "Show - just - lovely" , "Just; beautiful; blond" , "Average" , "Reasonable; pretty" , "Free from discrimination" .)

I've seen this clue in The Irish Times.

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