A belt -- all there is to keep miner's leader on pithead (3,6)
I believe the answer is:
sam browne
'a belt' is the definition.
(I know that Sam Browne is a type of belt)
'all there is to keep miner's leader on pithead' is the wordplay.
'all there' becomes 'sane' ('not all there' can mean insane).
'is to keep' is an insertion indicator.
'leader' indicates taking the first letters.
'on pithead' becomes 'brow' (I can't explain this - if you can you should give a lot more credence to this answer).
The initial letter of 'miners' is 'm'.
'm'+'brow'='mbrow'
'sane' placed around 'mbrow' is 'SAM BROWNE'.
Can you help me to learn more?
(Other definitions for sam browne that I've seen before include "Military officer's belt with shoulder strap" , "Military leather belt" , "military wear" .)