One time to limit sound of contentment that will rise and suddenly grow (6)

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

I (0ne) + T (time) round a reversal (that will rise) of PURR (sound of contentment) – I queried the definition here, knowing it as to attack / invade suddenly, as in Chambers but Collins adds ‘(of a population) to increase suddenly and greatly’