Sin about to turn up in twelfth month, turning up in groups (9)
I believe the answer is:
clustered
'twelfth month turning up in groups' is the definition.
I can't tell whether this definition defines the answer.
'sin about to turn up' is the wordplay.
'sin' becomes 'lust' (lust is one of the seven deadly sins).
'about' means one lot of letters goes inside another.
'to turn up' becomes 'cered' (I can't justify this - if you can you should believe this answer much more).
'lust' inserted inside 'cered' is 'CLUSTERED'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for clustered that I've seen before include "Grouped together" , "Bunched" .)