Herb the graduate is turned 50 (5)
I believe the answer is:
basil
'herb' is the definition.
(basil is a herb)
'graduate is turned 50' is the wordplay.
'graduate' becomes 'BA' (someone with a Bachelor of Arts degree).
'turned' is a reversal indicator.
'50' becomes 'l' (Roman numeral).
'is' backwards is 'si'.
'ba'+'si'+'l'='BASIL'
'the' acts as a link.
(Other definitions for basil that I've seen before include "A herb; --- Brush, Fawlty" , "Sweet-smelling herb" , "Saint" , "comic character" , "Aromatic herb with purplish-green leaves" .)