Excitedly say “wow” at finding hidden traveller (8)
I believe the answer is:
stowaway
'hidden traveller' is the definition.
(eg traveller hidden on a ship)
'excitedly say wow at' is the wordplay.
'excitedly' indicates an anagram (letters excited or stimulated into a new form).
'say'+'wow'+'at'='saywowat'
'saywowat' anagrammed gives 'STOWAWAY'.
'finding' is the link.
(Other definitions for stowaway that I've seen before include "One hiding aboard" , "Concealed traveller" , "Hider on board" , "Free rider" , "Illicit passenger on boat, plane etc" .)