Hard and fast going round tunnel (5)
I believe the answer is:
shaft
'tunnel' is the definition.
(thesaurus)
'hard and fast going round' is the wordplay.
'hard' becomes 'h'.
'and' says to put letters next to each other (I've seen this in other clues).
'going round' is an anagram indicator.
'h' after 'fast' is 'fasth'.
'fasth' with letters rearranged gives 'SHAFT'.
(Other definitions for shaft that I've seen before include "Ray (of sunlight)" , "Stem, handle; (light) ray" , "Spear handle; ray of sun" , "Ray of light; treat unfairly" , "Vertical mine access" .)