Some shown up in bunker (4)
I believe the answer is:
trap
'bunker' is the definition.
('trap' can be a synonym of 'bunker')
'some shown up' is the wordplay.
'some' becomes 'part'.
'shown up' shows that the letters should be reversed in order.
'part' written backwards gives 'TRAP'.
'in' is the link.
(Other definitions for trap that I've seen before include "Light horse-drawn carriage" , "website?" , "Catch; two-wheeled carriage" , "Snaring device" , "Snare - mouth" .)