Alternative way out of tunnel (3)
I believe the answer is:
sub
'alternative' is the definition.
(thesaurus)
'way out of tunnel' is the wordplay.
'way' becomes 's' (Compass direction - South).
'out of tunnel' becomes 'ub' (I can't explain this - if you can you should give a lot more credence to this answer).
's'+'ub'='SUB'
Can you help me to learn more?
(Other definitions for sub that I've seen before include "Prefix meaning beneath" , "Type of sandwich" , "Beneath on bus" , "Warship; membership fee" , "Briefly, membership fee or stand-in" .)