Get back on the subject of judge (6)
I believe the answer is:
redeem
'get back' is the definition.
(I've seen this before)
'the subject of judge' is the wordplay.
'the subject of' becomes 're' ('re' means regarding or about).
'judge' becomes 'deem' (I've seen this before).
're'+'deem'='REDEEM'
'on' acts as a link.
(Other definitions for redeem that I've seen before include "Repay (a bond); save" , "Buy black, or convert into cash" , "Save from sins or buy back" , "Buy back from the pawn shop" , "Retrieve from hell or pawn" .)