The text is a humorous twist on the idiom 'It ain't over till the fat lady sings', which means that one should not presume to know the outcome of an event until it has actually happened. The humor comes from the anticipation of the 'fat lady' preparing to sing (clearing her throat), suggesting the end is near, but it does not involve a play on words or similar-sounding words.
It ain ' t over , but the fat lady is clearing her throat .