Eyes Mark endlessly, before Gregory it was Julian! (4)
I believe the answer is:
year
'before gregory it was julian' is the definition.
I can't judge whether this defines the answer.
'eyes mark endlessly' is the wordplay.
'eyes' becomes 'ye' (I can't explain this - if you can you should believe this answer much more).
'endlessly' means to look at the middle letters (the ends of the words are removed).
The middle of 'mark' is 'ar'.
'ye'+'ar'='YEAR'
Can you help me to learn more?
(Other definitions for year that I've seen before include "See 33 Across" , "Group of students" , "Manage" , "26 fortnights" , "1381 say" .)