An elegiac broadside is A MOURNFUL, PLAINTIVE BALLAD
An elegy is a poem of lamentation to mourn the loss of something or someone. And broadside can mean a verbal attack. So the definition, a mournful, plaintive ballad fits best here.
The quotation that shows Jacques in a positive light is : "he was supposed to be sensual and a fast liver. It was therefore by the mere grace of the form that he was at first captured"