Small vessels in which one crosses over river (8)

'small vessels' is the definition.
(an ampoule is a small glass container)

'one crosses over river' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'crosses' becomes 'mules' (a mule is a cross between a horse and a donkey).
'over' indicates putting letters inside.
'river' becomes 'Po' (Italian river).
'mules' placed around 'po' is 'mpoules'.
'a'+'mpoules'='AMPOULES'

'in which' acts as a link.