Young Jack starts to use vessel on European river (8)
I believe the answer is:
juvenile
'young' is the definition.
(thesaurus)
'jack starts to use vessel on european river' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'starts to' suggests taking the first letters.
'on' is a charade indicator (letters next to each other).
'european' becomes 'e' (abbreviation e.g. EU).
'river' becomes 'nile' (Nile is an example**).
The first letters of 'use vessel' is 'uv'.
'j'+'uv'+'e'+'nile'='JUVENILE'
(Other definitions for juvenile that I've seen before include "Youth" , "Childish; youngster" , "Young or childish, sometimes delinquent" , "Minor" , "Infantile" .)