'Titanic' is romantic and also terrible (12)
I believe the answer is:
astronomical
'titanic' is the definition.
(thesaurus)
'romantic and also terrible' is the wordplay.
'and' is a charade indicator (letters next to each other).
'terrible' indicates anagramming the letters.
'romantic'+'also'='romanticalso'
'romanticalso' with letters rearranged gives 'ASTRONOMICAL'.
'is' is the link.
(Other definitions for astronomical that I've seen before include "Massive" , "Of the study of stars" , "Inconceivably large" , "Extremely large" , "Tremendous" .)