Fine to avoid long sentence — it’s a complete fiction (3)
I believe the answer is:
lie
'a complete fiction' is the definition.
'fine to avoid long sentence' is the wordplay.
'fine' becomes 'f' (abbreviation).
'to avoid' indicates named letters should be taken away.
'long sentence' becomes 'life'.
'life' with 'f' removed is 'LIE'.
'it's' acts as a link.
(Other definitions for lie that I've seen before include "Mistruth" , "Fabrication" , "Prevaricate or recline" , "False statement" , "habitat for animal or bird" .)