Betrayer turned, protecting second despot (4)
I believe the answer is:
tsar
'despot' is the definition.
(I've seen this before)
'betrayer turned protecting second' is the wordplay.
'betrayer' becomes 'rat' (synonyms).
'turned' shows that the letters should be reversed in order.
'protecting' is an insertion indicator (inserted letters are held or protected).
'second' becomes 's' (common abbreviation).
'rat' back-to-front is 'tar'.
'tar' going around 's' is 'TSAR'.
(Other definitions for tsar that I've seen before include "Old leader" , "Russian leader" , "Alexander and Nicholas held this title in Russia" , "Nicholas II was the last" , "One associated with 26 [KREMLIN]" .)