Joy after king ultimately reunites sons? (9)
I believe the answer is:
relations
'sons?' is the definition.
(son is a kind of relation)
'joy after king ultimately reunites' is the wordplay.
'joy' becomes 'elation' (elation is a kind of joy).
'after' means one lot of letters go next to another.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'ultimately' says to take the final letters.
The last letter of 'reunites' is 's'.
'elation' after 'r' is 'relation'.
'relation'+'s'='RELATIONS'
(Other definitions for relations that I've seen before include "kinsfolk" , "accounts" , "liaison" , "Tale-telling" , "They're connected by blood or marriage" .)