Jaw's in a bit of machinery (4)
I believe the answer is:
chin
'jaw's' is the definition.
(I have seen 'Lower jaw' mean 'chin' so perhaps 'jaw' could also mean 'chin')
'bit of machinery' is the wordplay.
'bit of' says the answer is hidden in the clue.
'CHIN' can be found hidden inside 'machinery'.
'in a' is the link.
(Other definitions for chin that I've seen before include "Punch to the face" , "Prominent feature" , "hit" , "part of mug" , "Outstanding jaw part" .)