Immaculate story about extremely lousy trifle (4-5)
I believe the answer is:
lily-white
'immaculate' is the definition.
(lily-white can mean immaculate or pure)
'story about extremely lousy trifle' is the wordplay.
'story' becomes 'lie' (story can mean a fabrication).
'about' is an insertion indicator.
'extremely' suggests removing the centre (only the extremes of the word are used).
'trifle' becomes 'whit' (I've seen this in another clue).
'lousy' with its centre taken out is 'ly'.
'ly'+'whit'='lywhit'
'lie' enclosing 'lywhit' is 'LILY-WHITE'.
(Other definitions for lily-white that I've seen before include "Without blame" , "Pure - irreproachable" , "faultless" , "Absolutely pure" , "Totally innocent" .)