Nebraska's top pitcher is less old (5)
I believe the answer is:
newer
'less old' is the definition.
('new' can be the opposite of 'old' so perhaps 'newer' is reasonable for 'less old')
'nebraska's top pitcher' is the wordplay.
'top' says to take the initial letters (in a down clue, the first letter is top).
'pitcher' becomes 'ewer' (synonyms).
The first letter of 'nebraska' is 'n'.
'n'+'ewer'='NEWER'
'is' is the link.
(Other definitions for newer that I've seen before include "More recent" , "Fresher or more novel" , "Fresher, not as old" , "More modern" , "More up-to-date" .)