He's fit: man working in tunnel underground (9)
I believe the answer is:
mineshaft
'tunnel underground' is the definition.
(type of underground tunnel)
'he's fit man working' is the wordplay.
'working' indicates an anagram.
'hes'+'fit'+'man'='hesfitman'
'hesfitman' with letters rearranged gives 'MINESHAFT'.
'in' is the link.
(Other definitions for mineshaft that I've seen before include "not a feature of 4, 21 [OPENCAST MINING]" , "Vertical hole" , "one's got bored" .)