Do up about fifty which are aged (3)
I believe the answer is:
old
'are aged' is the definition.
(similar in meaning)
'do up about fifty' is the wordplay.
'up' indicates anagramming the letters (letters get chucked up).
'about' indicates putting letters inside.
'fifty' becomes 'l' (Roman numeral).
'do' anagrammed gives 'od'.
'od' enclosing 'l' is 'OLD'.
'which' acts as a link.
(Other definitions for old that I've seen before include "Former, previous" , "At an early stage - at a late stage" , "Advanced in years" , "No longer young" , "Vintage" .)