Clue: A celebration after morning engaged in this country dance (7)
Answer: GAMBADO
Explanation: Definition: dance
A.M. in G[reat] B[ritain] + A DO (party, celebration). I knew this word from Katisha's line in *The Mikado*: "I'll spoil … your gay gambado" (to rhyme with "Mikado", though she doesn't get to say the word)