Jack in sober group unknown hero (4)
I believe the answer is:
ajax
'hero' is the definition.
(I've seen this before)
'jack in sober group unknown' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'in' indicates putting letters inside.
'sober group' becomes 'aa' (short for Alcoholics Anonymous).
'unknown' becomes 'x' (algebra).
'j' placed into 'aa' is 'aja'.
'aja'+'x'='AJAX'
(Other definitions for ajax that I've seen before include "eleven from the Netherlands" , "Foreign team" , "It's quite often seen in loo" , "Hero of Greek mythology" , "Mythical Greek hero, who fought against Troy" .)