Second Greek character, intellectual and imposing (10)
I believe the answer is:
monumental
'imposing' is the definition.
(both can mean impressively large or important)
'second greek character intellectual' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'greek character' becomes 'nu' (letter from the Greek alphabet).
'intellectual' becomes 'mental' (associated in meaning).
'mo'+'nu'+'mental'='MONUMENTAL'
'and' is the link.
(Other definitions for monumental that I've seen before include "Great in importance" , "Enormous" , "Stupendous" , "Massive and lasting" , "Large and important" .)