Mine's buried in the beach with Slavic military discipline (4,3,6)
I believe the answer is:
spit and polish
'military discipline' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'mine's buried in the beach with slavic' is the wordplay.
'mine' becomes 'pit' (down the pit).
'buried in' means one lot of letters goes inside another (inserted letters get buried).
'the beach' becomes 'sand'.
'with' is a charade indicator (letters next to each other).
'slavic' becomes 'Polish' (Polish is an example).
'pit' placed within 'sand' is 'spitand'.
'spitand'+'polish'='SPIT AND POLISH'
Can you help me to learn more?
(Other definitions for spit and polish that I've seen before include "Immaculate (military) neatness" , "service maintenance" , "Careful procedures" , "Thorough polishing" , "(Military) attention to order and appearance" .)