Obscure punishment given to retreating soldiers (6)
I believe the answer is:
arcane
'obscure' is the definition.
(I've seen this before)
'punishment given to retreating soldiers' is the wordplay.
'punishment' becomes 'cane' (I've seen this before).
'given to' says to put letters next to each other.
'retreating' is a reversal indicator.
'soldiers' becomes 'ra' (abbreviation for Royal Artillery).
'ra' backwards is 'ar'.
'cane' put after 'ar' is 'ARCANE'.
(Other definitions for arcane that I've seen before include "Ace ran, being mysterious" , "Mysterious, understood by few" , "Esoteric, mysterious" , "deep" , "Mysterious like a crane" .)