Because flesh is weak? (7)

I believe the answer is:

selfish

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'weak?' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

'because flesh is' is the wordplay.
'flesh is' anagrams to 'SELFISH'.
However, I'm unsure how this is indicated.

Can you help me to learn more?

(Other definitions for selfish that I've seen before include "Concerned only with one's personal welfare" , "Concerned only with oneself" , "Concerned chiefly with one's own interests" , "Ungenerous" , "Concerned only with self" .)

I've seen this clue in the Evening Standard.

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