Alien trapped in Alpine tunnel is a fool (9)
I believe the answer is:
simpleton
'a fool' is the definition.
(I've seen this before)
'alien trapped in alpine tunnel' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'trapped in' is an insertion indicator.
'alpine tunnel' becomes 'simplon' (I've seen this in another clue).
'et' inserted into 'simplon' is 'SIMPLETON'.
'is' is the link.
(Other definitions for simpleton that I've seen before include "Ninny" , "Dope" , "Ass" , "One very slow" , "Nincompoop" .)