A large, old nerd - a strange, great man (9)
I believe the answer is:
alexander
'great man' is the definition.
(Alexander the Great)
'a large old nerd a strange' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'old' becomes 'ex' (prefix meaning former or onetime).
'strange' indicates anagramming the letters.
'nerd'+'a'='nerda'
'nerda' is an anagram of 'ander'.
'a'+'l'+'ex'+'ander'='ALEXANDER'
(Other definitions for alexander that I've seen before include "Healing process based on improving posture" , "King of Macedon, conqueror of Greece, Egypt and Persia, d. 323 BC" , "Conquering Macedonian" , "A great Greek ruler?" , "Name of Pope" .)