A judge unknown to imprison a one-time war hero (4)
I believe the answer is:
ajax
'one-time war hero' is the definition.
(hero in Greek myth)
'a judge unknown to imprison a' is the wordplay.
'judge' becomes 'J' (abbreviation for judge).
'unknown' becomes 'x' (algebra).
'to imprison' is an insertion indicator.
'j'+'x'='jx'
'jx' placed around 'a' is 'jax'.
'a'+'jax'='AJAX'
(Other definitions for ajax that I've seen before include "Ancient Greek hero" , "side" , "eleven from the Netherlands" , "Strong Gr. hero (Iliad)" , "Greek warrior (Iliad)" .)