Sentence:  In the first example — "Dance revolutionised Burma" — you know the middle word is the <WSD>signpost</WSD> as "revolutionised" is too long to be the fodder (or letters to scramble). ... And bang, out jumps RUMBA. 
Meaning of the word marked with <WSD> tags:  A word or phrase within a clue that serves as an indicator, rather than being fodder.