Bill arriving in post for father of twelve (5)
I believe the answer is:
jacob
'father of twelve' is the definition.
(Biblical figure)
'bill arriving in post' is the wordplay.
'bill' becomes 'ac'.
'arriving in' means one lot of letters goes inside another.
'post' becomes 'job' (I've seen this before).
'ac' put into 'job' is 'JACOB'.
'for' acts as a link.
(Other definitions for jacob that I've seen before include "Rachel's husband" , "Hebrew patriarch" , "Father of the twelve patriarchs of Israel" , "A son of Isaac (Bible)" , "Esau's twin brother" .)