Really old individual breaking leg (5-3)

I believe the answer is:

stone age

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'really old' is the definition.
The definition suggests a singular noun which matches the answer.

'individual breaking leg' is the wordplay.
'individual' becomes 'one' (an individual is one person).
'breaking' is an insertion indicator.
'leg' becomes 'stage' (synonyms).
'one' placed within 'stage' is 'STONE-AGE'.

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(Other definitions for stone age that I've seen before include "Prehistoric period" , "Early epoch when man used pre-metal tools" , "Pre-historic era" , "before copper came in" , "Early period" .)

I've seen this clue in The Times.

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