Female in post office sorts life classes (10)

I believe the answer is:

phenotypes

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'life classes' is the definition.
The answer and definition can be both human attributes as well as being plural nouns.
Perhaps there's an association between them I don't understand?

'female in post office sorts' is the wordplay.
'female' becomes 'hen' (female chicken).
'in' is an insertion indicator.
'post office' becomes 'po' (abbreviation).
'sorts' becomes 'types' (type is a kind of sort).
'hen' put into 'po' is 'pheno'.
'pheno'+'types'='PHENOTYPES'

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I've seen this clue in The Irish Times.

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