Happen to be our inclusion of a double century (5)
I believe the answer is:
occur
'happen to be' is the definition.
(thesaurus)
'our inclusion of a double century' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is found in the answer.
'century' could be 'c' (common abbreviation) and 'c' is found in the answer.
The remaining letters 'cur' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for occur that I've seen before include "found" , "Spring to mind" , "Happen, take place" , "crop up" , "Come to pass" .)