Support Greek character, last to ascend to top (4)
I believe the answer is:
abet
'support' is the definition.
(I've seen this in another clue)
'greek character last to ascend to top' is the wordplay.
'greek character last' becomes 'bet' (I am not sure about this - if you are sure you should believe this answer much more).
'to' says to put letters next to each other (I've seen this in other clues).
'to top' indicates taking the first letters (in a down clue, the first letter is top).
The initial letter of 'ascend' is 'a'.
'bet' after 'a' is 'ABET'.
Can you help me to learn more?
(Other definitions for abet that I've seen before include "Encourage or assist (someone) to commit crime" , "Incite or encourage" , "Incite (to commit an offence)" , "Beat (anag.)" , "Help to do something wrong" .)