“Long” means more than twelve months (5)
I believe the answer is:
yearn
'long' is the definition.
('yearn' can be a synonym of 'long')
'means more than twelve months' is the wordplay.
'means' becomes 'n' (I can't justify this - if you can you should give a lot more credence to this answer).
'more than' says to put letters next to each other.
'twelve months' becomes 'year' (I've seen this before).
'n' after 'year' is 'YEARN'.
Can you help me to learn more?
(Other definitions for yearn that I've seen before include "Mark of injury" , "Long - pine" , "Have strong desire" , "Hanker" , "Have a craving" .)