Trip in upturned bunker test (4)
I believe the answer is:
trek
'trip' is the definition.
(I know that trek is a type of trip)
'in upturned bunker test' is the wordplay.
'upturned' says the letters should be written in reverse.
'in' says the answer is hidden in the clue.
'kert' is hidden within 'bunker test'.
'kert' back-to-front is 'TREK'.
(Other definitions for trek that I've seen before include "Long hard journey on foot" , "A long arduous walk" , "Long difficult walk" , "Footslog" , "A long arduous march" .)