Look inside tunnel, going up (4)
I believe the answer is:
ogle
'look' is the definition.
(I know that ogle is a type of look)
'inside tunnel going up' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'elgo' is hidden in the letters of 'tunnel going'.
'elgo' back-to-front is 'OGLE'.
(Other definitions for ogle that I've seen before include "Gawp" , "Give the glad eye" , "Lasciviously look" , "Lecherous look" , "Make eyes at" .)