Iron ore, essentially, loaded on board transport vessel (3-5)

'vessel' is the definition.
(car-ferry is a kind of vessel)

'iron ore essentially loaded on board transport' is the wordplay.
'iron' becomes 'Fe' (chemical symbol for iron).
'essentially' indicates the central letters.
'loaded' means one lot of letters go next to another.
'on board' means one lot of letters goes inside another (inserted letters are taken on board).
'transport' becomes 'carry' (synonyms).
The central letter of 'ore' is 'r'.
'fe'+'r'='fer'
'fer' inserted into 'carry' is 'CAR-FERRY'.