Wow, a drunk in sojourn — illegal traveller! (8)
I believe the answer is:
stowaway
'illegal traveller' is the definition.
(I've seen this before)
'wow a drunk in sojourn' is the wordplay.
'drunk' indicates an anagram.
'in' is an insertion indicator.
'sojourn' becomes 'stay' (I've seen this before).
'wow'+'a'='wowa'
'wowa' is an anagram of 'owaw'.
'owaw' put into 'stay' is 'STOWAWAY'.
(Other definitions for stowaway that I've seen before include "Hider on board" , "One in a hold" , "One hiding on ship" , "Unauthorised passenger" , "Travel secretly" .)