Tyrant is, or could be, short-lived (10)
I believe the answer is:
transitory
'short-lived' is the definition.
(synonyms)
'tyrant is or could be' is the wordplay.
'could be' indicates an anagram.
'tyrant'+'is'+'or'='tyrantisor'
'tyrantisor' is an anagram of 'TRANSITORY'.
(Other definitions for transitory that I've seen before include "Here-today-gone-tomorrow" , "Passing" , "Fleeting" , "Short-lived" , "Ephemeral" .)