Keep long dissertation's opening short somehow (10)
I believe the answer is:
stronghold
'keep' is the definition.
(thesaurus)
'long dissertation's opening short somehow' is the wordplay.
'opening' indicates taking the first letters.
'somehow' indicates an anagram.
The initial letter of 'dissertations' is 'd'.
'long'+'d'+'short'='longdshort'
'longdshort' anagrammed gives 'STRONGHOLD'.
(Other definitions for stronghold that I've seen before include "Keep" , "castle" , "Fortress" , "Defence" .)