Young adult going short for a very long time (9)
I believe the answer is:
millennia
'a very long time' is the definition.
'young adult going short' is the wordplay.
'young adult' becomes 'millennial'.
'going short' means to remove the last letter.
'millennial' with its last letter taken away is 'MILLENNIA'.
'for' is the link.
(Other definitions for millennia that I've seen before include "ages" , "2000 years minimum" , "Thousands of years" , "Thousand-year periods" , "Thousand year periods" .)