Bogus leading lady once remained unknown (6)
I believe the answer is:
ersatz
'bogus' is the definition.
(unreal or false)
'leading lady once remained unknown' is the wordplay.
'leading lady once' becomes 'ER' (short for Elizabeth Regina ie Queen Elizabeth).
'remained' becomes 'sat' (I've seen this in another clue).
'unknown' becomes 'z' (represents an unknown value in algebra).
'er'+'sat'+'z'='ERSATZ'
(Other definitions for ersatz that I've seen before include "False" , "Inferior substitute" , "Imitation (from Ger.)" , "Mock" , "Artificial, imitation - from German" .)