To 'clown around' means to behave in a silly, immature or irresponsible way. If you accuse someone of 'clowning around' it is similar to saying they are 'messing' or not taking things seriously. This is an expression more often used in a social context to describe someone's behaviour but you could hear it in professional contexts also often as an insult. "He has been accused of 'clowning around' at politics and not taking the matter seriously." Similar meaning expressions and phrases include 'messing around' 'play the fool' and 'horse around'.