Showing regularity unknown at the beginning (6)
I believe the answer is:
yearly
'showing regularity unknown at the beginning' is the definition.
I can't tell whether this defines the answer.
'unknown at the beginning' is the wordplay.
'unknown' becomes 'y' (algebra).
'at the beginning' becomes 'early' (I can't explain this - if you can you should give a lot more credence to this answer).
'y'+'early'='YEARLY'
Can you help me to learn more?
(Other definitions for yearly that I've seen before include "Every 12 months" , "when Christmas comes round" , "on a regular basis" , "in each revolution?" , "Annually" .)