Post mineral box (4)
I believe the answer is:
spar
This might be a double definition.
'post mineral' is the first definition.
'spar' can be an answer for 'mineral' (spar is a kind of mineral). I'm not certain of the 'post' bit.
'box' is the second definition.
(sparring is a kind of boxing)
Can you help me to learn more?
(Other definitions for spar that I've seen before include "What boxers do" , "Shadow box" , "Box gently; main beam of aircraft wing" , "Round pole to support sail" , "Raps with wooden beam" .)