Clue: Cheerful Boatman's a mischievous chap, prone to compileritis? (2,7)
Answer: UP ONESELF
Explanation: Definition: prone to compileritis
UP (cheerful) + ONE'S (Boatman's) + ELF – a bit of an in-joke, "compileritis" being many a word used on here by former commenter hedgehoggy to refer to "showing off" by setters