Is this a metaphor?

She wanted to tell him to stop being a chicken, because that was what it came down to when you cut to the chase, as their Dad said.

She wanted to tell him to stop being a chicken ...

I guess you could say so!
http://www.lausd.k12.ca.us/lausd/resources/Literary.Terms.2.html#Metaphor

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Yes, this sentence contains a metaphor. A metaphor is a figure of speech in which a word or phrase is used to represent something else, suggesting a comparison or similarity. In this case, the phrase "stop being a chicken" is not meant to be taken literally, but rather as a metaphor for encouraging someone to be brave and face their fears. The speaker is comparing the person's behavior to that of a chicken, implying that they are being timid or afraid.