Acknowledge first of miners in access shaft (5)
I believe the answer is:
admit
'acknowledge' is the definition.
(synonyms)
'first of miners in access shaft' is the wordplay.
'first of miners' becomes 'm' (ast letter of 'miners').
'in' is an insertion indicator.
'access shaft' becomes 'adit' (I can't explain this - if you can you should believe this answer much more).
'm' inserted inside 'adit' is 'ADMIT'.
Can you help me to learn more?
(Other definitions for admit that I've seen before include "Usher in" , "Confess and permit entrance" , "Confess - acknowledge" , "receive" , "Recognise" .)