Yankees entertain peer every twelve months (6)
I believe the answer is:
yearly
'every twelve months' is the definition.
(I've seen this before)
'yankees entertain peer' is the wordplay.
'yankees' becomes 'yy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'entertain' means one lot of letters goes inside another.
'peer' becomes 'earl' (earl is a kind of peer).
'yy' going around 'earl' is 'YEARLY'.
Can you help me to learn more?
(Other definitions for yearly that I've seen before include "Happening every twelve months" , "Annual periodical" , "Every 365 or 366 days" , "A hundred times a century" , "Unpleasant noise" .)