Shaft, access directly into tunnel, first of all (4)
I believe the answer is:
adit
'shaft' is the definition.
(thesaurus)
'access directly into tunnel first of all' is the wordplay.
'first of all' says to take the initial letters.
The first letters of 'access directly into tunnel' is 'ADIT'.
(Other definitions for adit that I've seen before include "Mine entrance passage" , "Mine opening" , "Almost horizontal mine shaft" , "Means of access to mine" , "Mine access passage" .)