Clue: Celebration of son getting back ending finally after one (7)
Answer: SHINDIG
Explanation: Definition: Celebration
 S(abbrev. for "son") **plus**(getting) HIND(situated at the back part of the body) + [ **last letter of**(… finally) "ending" **placed after**(after) I(Roman numeral for "one") ].
Defn: A lively party as a celebration.