Piece of armour over in mine soldiers left (7)

'piece' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'armour over in mine soldiers left' is the wordplay.
'armour over' becomes 'o' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'mine' becomes 'pit' (down the pit).
'soldiers' becomes 're' (Royal Engineers).
'left' becomes 'l' (common abbreviation).
'o' going into 'pit' is 'poit'.
'poit'+'re'+'l'='POITREL'

'of' is the link.

Can you help me to learn more?