Keep short introduction to dreadfully long novel (10)
I believe the answer is:
stronghold
'keep' is the definition.
(a military stronghold)
'short introduction to dreadfully long novel' is the wordplay.
'introduction to' says to take the initial letters.
'novel' indicates anagramming the letters (letters in a new or novel order).
The first letter of 'dreadfully' is 'd'.
'short'+'d'+'long'='shortdlong'
'shortdlong' anagrammed gives 'STRONGHOLD'.
(Other definitions for stronghold that I've seen before include "Fortress" , "castle" , "Keep" , "Defence" .)