Guerrilla misbehaving in Sparta (8)
I believe the answer is:
partisan
'guerrilla' is the definition.
(I've seen this before)
'misbehaving in sparta' is the wordplay.
'misbehaving' indicates anagramming the letters.
'in'+'sparta'='insparta'
'insparta' anagrammed gives 'PARTISAN'.
(Other definitions for partisan that I've seen before include "Enthusiast" , "Strongly supporting one side" , "(One) taking one side in dispute" , "Old battle-axe" , "Unquestioning supporter -- aspirant (anag)" .)