Militaristic state’s ability in spring (6)
I believe the answer is:
sparta
'militaristic state's' is the definition.
(city-state in ancient Greece)
'ability in spring' is the wordplay.
'ability' becomes 'art' (art is a kind of ability).
'in' indicates putting letters inside.
'spring' becomes 'spa' (spa can mean a mineral spring).
'art' placed inside 'spa' is 'SPARTA'.
(Other definitions for sparta that I've seen before include "Old Greek city state, famous for its citizens' austere lifestyle" , "Ancient Greek city whose citizens were famed for toughness" , "Ancient Greek city noted for its austerity" , "austere place" , "Laconic state" .)