"It's about time!" (6)
'it's about' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for circa that I've seen before include "Approximately (Latin)" , "Preceding a date, this means about" , "Roughly" , "Approximately (of a year)" , "Approximately, around" .)
(Another definition for atlast that I've seen is " ultimately".)