Explosives on lid covering opening of first access tunnel (9)

'tunnel' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?

'explosives on lid covering opening of first access' is the wordplay.
'explosives' becomes 'mines'.
'on lid covering' becomes 'hat' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'opening of' indicates taking the first letters.
'access' means one lot of letters goes inside another (inserted letters gain access).
The initial letter of 'first' is 'f'.
'hat' enclosing 'f' is 'haft'.
'mines'+'haft'='MINESHAFT'

Can you help me to learn more?