Lacking a timely qualification (8)

I believe the answer is:

underage

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'lacking' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

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

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(Other definitions for underage that I've seen before include "Need a rug (anag.)" , "Not legally old enough" , "Not yet eligible to vote - too young" , "Not old enough to exercise legal right" , "Too tender" .)

I've seen this clue in The Globe and Mail.

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