Emperor Tiberius, sombre and reclusive for starters (4)
I believe the answer is:
tsar
'emperor' is the definition.
(I've seen this before)
'tiberius sombre and reclusive for starters' is the wordplay.
'for starters' indicates taking the first letters.
The first letters of 'tiberius sombre and reclusive' is 'TSAR'.
(Other definitions for tsar that I've seen before include "Nicholas II, say" , "Nicholas II was the last" , "Peter the Great, for example" , "Despot" , "Title given to the kings of Bulgaria" .)