Reported onslaught by the convent in isolated mountain peak (7)

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

Sound-alike (reported) of NUN ATTACK (an onslaught by the members of a convent). A mountain peak surrounded by ice. The dictionaries disagree on whether the first syllable should rhyme with “sun” or “soon”, but the clue works better with the first of these.