Clue: So prone to change, his slips provide amusement (7)
Answer: SPOONER
Explanation: So prone to change, his slips provide amusement (7)
*(so prone) 
*Referring, of course, to Crosswordland's favourite Oxford don, William Archibald Spooner, whose tendency to get his syllables mixed up added the word "spoonerism" to our vocabulary.