Brief novel penned by fifth columnist (6)
I believe the answer is:
skimpy
'brief' is the definition.
(skimpy means short or brief)
'novel penned by fifth columnist' is the wordplay.
'novel' becomes 'kim' (Rudyard Kipling novel).
'penned by' is an insertion indicator (inserted letters are penned in).
'fifth columnist' becomes 'spy' (a fifth columnist helps enemy nations).
'kim' placed inside 'spy' is 'SKIMPY'.
(Other definitions for skimpy that I've seen before include "'Scanty, barely enough (6)'" , "Scanty (clothing)" , "Having hardly enough" , "Scanty or mean" , ", naughty thing, the blue skirts?" .)