Allow machine head inside mine shaft (5)
I believe the answer is:
admit
'allow' is the definition.
(admit can mean to allow or accept)
'machine head inside mine shaft' is the wordplay.
'head' says to take the initial letters.
'inside' is an insertion indicator.
'mine shaft' becomes 'adit' (I've seen this before).
The first letter of 'machine' is 'm'.
'm' going into 'adit' is 'ADMIT'.
(Other definitions for admit that I've seen before include "Confess (to something)" , "Allow entry" , "Disclose" , "Confess; allow in" , "Agree to the truth" .)