One-sided fighting in Sparta (8)
I believe the answer is:
partisan
'one-sided' is the definition.
(similar in meaning)
'fighting in sparta' is the wordplay.
'fighting' is an anagram indicator.
'in'+'sparta'='insparta'
'insparta' anagrammed gives 'PARTISAN'.
(Other definitions for partisan that I've seen before include "Old battle-axe" , "of prejudiced views" , "Happening immediately" , "Unquestioning supporter -- aspirant (anag)" , "Enthusiast" .)