Long time after old Papa Haydn's works? (5)

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

O (old), P (papa – NATO alphabet), ERA (long time). Joseph Haydn’s operas tend to have been neglected over the years by comparison with much of his other output, but he wrote a couple of dozen. The surface is apt because the affectionate nickname ‘Papa’ was given to Haydn during his time at the court of Esterházy. Please see Wil Ransome’s comment below for an alternative interpretation of the definition.