Clue: Was familiar with, reportedly, parameters of Mornington Crescent (3,4)
Answer: NEW MOON
Explanation: Definition: Crescent
A charade of NEW, sounding like ('reportedly') KNEW ('was familiar with') plus MOON ('parameters of'**MO**rningt**ON**' the use of 'parameters' raised an eyebrow; the nearest I can see is that it can mean limits).