A second son, one behaving foolishly (3)

I believe the answer is:

ass

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'one behaving foolishly' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'a second son' is the wordplay.
'second' becomes 's' (common abbreviation).
'son' becomes 's'.
'a'+'s'+'s'='ASS'

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(Other definitions for ass that I've seen before include "Animal related to horses" , "Animal of horse family" , "Quadruped; fool" , "An American backside" , "Small hoofed animal" .)

I've seen this clue in The Times.

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