Character from Titanic holding up duck (5)
I believe the answer is:
omega
'character' is the definition.
(letter of the Greek alphabet)
'titanic holding up duck' is the wordplay.
'titanic' becomes 'mega' (both can mean huge).
'holding up' means one lot of letters go next to another (some letters go underneath or hold up others).
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'mega' after 'o' is 'OMEGA'.
'from' is the link.
(Other definitions for omega that I've seen before include "Foreign character" , "16 letter" , "Ultimate letter" , "Greek character symbolizing resistance" , "character from Athens" .)