Explain these won’t ya?

With which condition one doesn’t normally feel great? (11)

megalomania

Bloom reserving different name for female who knows best (6,6)

Mother Nature

Cold War aftermath? Tough to secure outright victory on retreating! (7,6)

Nuclear Winter

Social networker using rather cute, brief epithet, say (7)

Tweeter

On the drink heading to North Country (5)

Niger