Not as old as singer-songwriter getting on in retirement (7)

I believe the answer is:

younger

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'not as old' is the definition.
('young' can be the opposite of 'old' so perhaps 'younger' is reasonable for 'not as old')

I cannot understand how the remainder of the clue works.

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(Other definitions for younger that I've seen before include "Less aged" , "Less grey" , "more boyish" , "More youthful" , "Not so old" .)

I've seen this clue in The Guardian.

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