Short letter rejecting new fiction (3)
I believe the answer is:
lie
'fiction' is the definition.
(I've seen this before)
'short letter rejecting new' is the wordplay.
'short letter' becomes 'line' (synonyms).
'rejecting' is a deletion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'line' with 'n' taken out is 'LIE'.
(Other definitions for lie that I've seen before include "Rest; deception" , "Fib - be prone" , "Story" , "A white one is considered harmless" , "Commit perjury" .)