Endorse first Latin novel about the start of the Interregnum (7)
I believe the answer is:
initial
'endorse' is the definition.
(thesaurus)
'first latin novel about the start of the interregnum' is the wordplay.
'first' becomes 'i' (I in Roman numerals eg Elizabeth I**).
'novel' indicates anagramming the letters.
'about' is an insertion indicator.
'the start of' indicates taking the first letters.
The first letter of 'interregnum' is 'i'.
'latin' is an anagram of 'ntial'.
'ntial' placed around 'i' is 'nitial'.
'i'+'nitial'='INITIAL'
(Other definitions for initial that I've seen before include "E in Eileen" , "subscribe, minimally" , "First - opening" , "Letter - opening" , "Beginning - first letter" .)