First empire almost to be destroyed (5)
I believe the answer is:
prime
'first' is the definition.
(similar in meaning)
'empire almost to be destroyed' is the wordplay.
'almost' means to remove the first letter.
'to be destroyed' is an anagram indicator.
'empire' with its first letter taken off is 'mpire'.
'mpire' with letters rearranged gives 'PRIME'.
(Other definitions for prime that I've seen before include "Original" , "Chief or most important, like a minister" , "Foremost" , "Prepare to set off" , "Brief" .)