Resistance fighter trained in Sparta (8)
I believe the answer is:
partisan
'resistance fighter' is the definition.
(thesaurus)
'trained in sparta' is the wordplay.
'trained' is an anagram indicator (to train can mean to grow something into a shape).
'in'+'sparta'='insparta'
'insparta' with letters rearranged gives 'PARTISAN'.
(Other definitions for partisan that I've seen before include "Devotee of a cause" , "Strong party supporter" , "In Sparta (anag)" , "One-sided" , "Unfair" .)