Run away from tunnel opening (5)
I believe the answer is:
elope
'run away' is the definition.
(I've seen this before)
'from tunnel opening' is the wordplay.
'from' says the answer is hidden in the clue.
'ELOPE' can be found hidden inside 'tunnel opening'.
(Other definitions for elope that I've seen before include "Do a runner" , "Run away with one's beloved" , "Run away with lover" , "Bolt" , "leave with lover" .)