Spoonerisms are created by transposing letters in different words to make new words, e.g. Mum and Dad become dumb and mad.
Being vocal, Spoonerisms are sometimes called tips of the slung (slips of the tongue). The same sounds can have many different spellings in English, so grilled beef could become billed grief.
Spoonerisms are sometimes used in cryptic clues and usually indicated by the name Spooner.
Basic spoonerisms - letter switch