An L of an age? (5)

I believe the answer is:

fifty

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'an l' is the definition.
('fifty' can be a synonym of 'l')

I don't understand how the remainder of the clue works.

(Other definitions for fifty that I've seen before include "Half-century" , "L" , "A multiple of ten" .)

I've seen this clue in the Evening Standard.

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