Happiness with Queen Mary, perhaps, after king's affair (12)
I believe the answer is:
relationship
'king's affair' is the definition.
'relationship' can be an answer for 'affair' (I've seen this before). I am not certain of the 'king's' bit.
'happiness with queen mary perhaps' is the wordplay.
'happiness' becomes 'elation' (elation can mean intense happiness).
'with' says to put letters next to each other.
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'mary perhaps' becomes 'ship' (I am not sure about this - if you are sure you should believe this answer much more).
'elation' after 'r' is 'relation'.
'relation'+'ship'='RELATIONSHIP'
'after' acts as a link.
Can you help me to learn more?
(Other definitions for relationship that I've seen before include "Romance" , "Intimate affair" , "emotional tie-up" , "Connection, bond" , "Linkage; affair" .)