Orchestral piece informally - a rich source containing heart of Elgar on reflection (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A MINE (as in “a mine of information” = a rich source), containing the middle letter (heart) of [el] G [ar], all reversed (on reflection). Extended definition: informal name for Elgar’s orchestral piece Enigma Variations , which contains musical portraits of the composer and several of his friends.