Marcus occasionally entertains noble relations (8)
I believe the answer is:
accounts
'relations' is the definition.
(account is a kind of relation)
'marcus occasionally entertains noble' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'entertains' indicates putting letters inside.
'noble' becomes 'count' (count is a type of aristocrat).
The alternating letters of 'marcus' are 'acs'.
'acs' going around 'count' is 'ACCOUNTS'.
(Other definitions for accounts that I've seen before include "Gives financial reckoning, or explains cause" , "bills" , "Statements of financial transactions" , "Rates" , "Records of financial transactions (8)" .)