Terrible triteness has the capacity to fascinate (9)
I believe the answer is:
interests
'the capacity to fascinate' is the definition.
I can't tell whether this defines the answer.
'terrible triteness' is the wordplay.
'terrible' is an anagram indicator.
'triteness' with letters rearranged gives 'INTERESTS'.
'has' is the link.
Can you help me to learn more?
(Other definitions for interests that I've seen before include "Advantages" , "Pastimes or concerns" , "Holds the attention" , "Hobbies or preoccupations" , "Activities or subjects which one enjoys" .)