Judge about to imprison German horseman (6)
I believe the answer is:
knight
'horseman' is the definition.
(knight on horseback)
'judge about to imprison german' is the wordplay.
'judge' becomes 'think' (thinking is a kind of judging).
'about' is a reversal indicator.
'to imprison' is an insertion indicator.
'german' becomes 'g' (abbreviation).
'think' written backwards gives 'kniht'.
'kniht' going around 'g' is 'KNIGHT'.
(Other definitions for knight that I've seen before include "Chesspiece which moves in the shape of an L" , "Gawain?" , "Holder of non-hereditary title in UK" , "Chess piece shaped like horse's head" , "Old horseman (on board?)" .)