A river boy reaching the very top (6)
I believe the answer is:
apogee
'the very top' is the definition.
(thesaurus)
'a river boy' is the wordplay.
'river' becomes 'Po' (Italian river).
'boy' becomes 'gee' (exclamations of surprise).
'a'+'po'+'gee'='APOGEE'
'reaching' is the link.
(Other definitions for apogee that I've seen before include "Apex, peak" , "crown" , "Final climactic stage" , "highly significant point" , "(Orbit) most distant point from earth" .)